Update golang.org/x/net/context to latest.

```
$ gvt delete golang.org/x/net/context
$ gvt fetch golang.org/x/net/context
2017/10/08 XX:XX:XX Fetching: golang.org/x/net/context
$ git grep -l "golang.org/x/net/context" | grep -v vendor | xargs sed -i 's:golang.org/x/net/context:context:g'
$ git grep -l "context/ctxhttp" | grep -v vendor | xargs sed -i 's:context/ctxhttp:golang.org/x/net/context/ctxhttp:g'
$ gofmt -s -w app
$ gofmt -s -w common
$ gofmt -s -w probe
$ gofmt -s -w prog
$ gofmt -s -w tools
```

This fixes errors like:
```
app/multitenant/aws_collector.go:222: cannot use func literal (type func("github.com/weaveworks/scope/vendor/golang.org/x/net/context".Context) error) as type func("context".Context) error in argument to instrument.TimeRequestHistogram
app/multitenant/aws_collector.go:439: cannot use func literal (type func("github.com/weaveworks/scope/vendor/golang.org/x/net/context".Context) error) as type func("context".Context) error in argument to instrument.TimeRequestHistogram
app/multitenant/memcache_client.go:155: cannot use func literal (type func("github.com/weaveworks/scope/vendor/golang.org/x/net/context".Context) error) as type func("context".Context) error in argument to instrument.TimeRequestHistogramStatus
app/multitenant/memcache_client.go:210: cannot use func literal (type func("github.com/weaveworks/scope/vendor/golang.org/x/net/context".Context) error) as type func("context".Context) error in argument to instrument.TimeRequestHistogramStatus
app/multitenant/s3_client.go:74: cannot use func literal (type func("github.com/weaveworks/scope/vendor/golang.org/x/net/context".Context) error) as type func("context".Context) error in argument to instrument.TimeRequestHistogram
app/multitenant/s3_client.go:91: cannot use func literal (type func("github.com/weaveworks/scope/vendor/golang.org/x/net/context".Context) error) as type func("context".Context) error in argument to instrument.TimeRequestHistogram
app/multitenant/sqs_control_router.go:99: cannot use func literal (type func("github.com/weaveworks/scope/vendor/golang.org/x/net/context".Context) error) as type func("context".Context) error in argument to instrument.TimeRequestHistogram
app/multitenant/sqs_control_router.go:133: cannot use func literal (type func("github.com/weaveworks/scope/vendor/golang.org/x/net/context".Context) error) as type func("context".Context) error in argument to instrument.TimeRequestHistogram
app/multitenant/sqs_control_router.go:163: cannot use func literal (type func("github.com/weaveworks/scope/vendor/golang.org/x/net/context".Context) error) as type func("context".Context) error in argument to instrument.TimeRequestHistogram
app/multitenant/sqs_control_router.go:199: cannot use func literal (type func("github.com/weaveworks/scope/vendor/golang.org/x/net/context".Context) error) as type func("context".Context) error in argument to instrument.TimeRequestHistogram
app/multitenant/sqs_control_router.go:199: too many errors
```

app/api_report.go

@@ -4,7 +4,7 @@ import (
 	"net/http"
 	"time"
 
-	"golang.org/x/net/context"
+	"context"
 
 	"github.com/weaveworks/scope/probe/host"
 	"github.com/weaveworks/scope/report"

app/api_topologies.go

@@ -1,6 +1,7 @@
 package app
 
 import (
+	"context"
 	"fmt"
 	"net/http"
 	"net/url"
@@ -11,7 +12,6 @@ import (
 
 	"github.com/gorilla/mux"
 	log "github.com/sirupsen/logrus"
-	"golang.org/x/net/context"
 
 	"github.com/weaveworks/scope/probe/docker"
 	"github.com/weaveworks/scope/probe/kubernetes"

app/api_topology.go

@@ -1,12 +1,12 @@
 package app
 
 import (
+	"context"
 	"net/http"
 	"time"
 
 	"github.com/gorilla/mux"
 	log "github.com/sirupsen/logrus"
-	"golang.org/x/net/context"
 
 	"github.com/weaveworks/scope/common/xfer"
 	"github.com/weaveworks/scope/render"

app/benchmark_internal_test.go

@@ -7,7 +7,7 @@ import (
 	"testing"
 	"time"
 
-	"golang.org/x/net/context"
+	"context"
 
 	"github.com/weaveworks/scope/render"
 	"github.com/weaveworks/scope/report"

app/collector.go

@@ -9,7 +9,7 @@ import (
 	"sync"
 	"time"
 
-	"golang.org/x/net/context"
+	"context"
 
 	"github.com/weaveworks/common/mtime"
 	"github.com/weaveworks/scope/report"

app/collector_test.go

@@ -4,7 +4,7 @@ import (
 	"testing"
 	"time"
 
-	"golang.org/x/net/context"
+	"context"
 
 	"github.com/weaveworks/common/mtime"
 	"github.com/weaveworks/common/test"

app/control_router.go

@@ -5,7 +5,7 @@ import (
 	"math/rand"
 	"sync"
 
-	"golang.org/x/net/context"
+	"context"
 
 	"github.com/weaveworks/scope/common/xfer"
 )

app/controls.go

@@ -4,10 +4,10 @@ import (
 	"net/http"
 	"net/rpc"
 
+	"context"
 	"github.com/gorilla/mux"
 	log "github.com/sirupsen/logrus"
 	"github.com/ugorji/go/codec"
-	"golang.org/x/net/context"
 
 	"github.com/weaveworks/scope/common/xfer"
 )

app/multitenant/aws_collector.go

@@ -1,6 +1,7 @@
 package multitenant
 
 import (
+	"context"
 	"crypto/md5"
 	"fmt"
 	"io"
@@ -16,7 +17,6 @@ import (
 	"github.com/nats-io/nats"
 	"github.com/prometheus/client_golang/prometheus"
 	log "github.com/sirupsen/logrus"
-	"golang.org/x/net/context"
 
 	"github.com/weaveworks/common/instrument"
 	"github.com/weaveworks/scope/app"

app/multitenant/billing_emitter.go

@@ -7,9 +7,9 @@ import (
 	"strings"
 	"time"
 
+	"context"
 	log "github.com/sirupsen/logrus"
 	billing "github.com/weaveworks/billing-client"
-	"golang.org/x/net/context"
 
 	"github.com/weaveworks/scope/app"
 	"github.com/weaveworks/scope/report"

app/multitenant/consul_pipe_router.go

@@ -1,6 +1,7 @@
 package multitenant
 
 import (
+	"context"
 	"fmt"
 	"io"
 	"net/http"
@@ -10,8 +11,8 @@ import (
 
 	"github.com/gorilla/mux"
 	"github.com/gorilla/websocket"
+
 	log "github.com/sirupsen/logrus"
-	"golang.org/x/net/context"
 
 	"github.com/weaveworks/common/mtime"
 	"github.com/weaveworks/scope/app"

app/multitenant/consul_pipe_router_internal_test.go

@@ -9,7 +9,7 @@ import (
 	"sync"
 	"testing"
 
-	"golang.org/x/net/context"
+	"context"
 
 	"github.com/weaveworks/scope/app"
 	"github.com/weaveworks/scope/common/xfer"

app/multitenant/memcache_client.go

@@ -1,6 +1,7 @@
 package multitenant
 
 import (
+	"context"
 	"fmt"
 	"net"
 	"sort"
@@ -9,8 +10,8 @@ import (
 
 	"github.com/bradfitz/gomemcache/memcache"
 	"github.com/prometheus/client_golang/prometheus"
+
 	log "github.com/sirupsen/logrus"
-	"golang.org/x/net/context"
 
 	"github.com/weaveworks/common/instrument"
 	"github.com/weaveworks/scope/report"

app/multitenant/s3_client.go

@@ -3,11 +3,11 @@ package multitenant
 import (
 	"bytes"
 
+	"context"
 	"github.com/aws/aws-sdk-go/aws"
 	"github.com/aws/aws-sdk-go/aws/session"
 	"github.com/aws/aws-sdk-go/service/s3"
 	"github.com/prometheus/client_golang/prometheus"
-	"golang.org/x/net/context"
 
 	"github.com/weaveworks/common/instrument"
 	"github.com/weaveworks/scope/report"

app/multitenant/sqs_control_router.go

@@ -2,6 +2,7 @@ package multitenant
 
 import (
 	"bytes"
+	"context"
 	"encoding/json"
 	"fmt"
 	"math/rand"
@@ -12,8 +13,8 @@ import (
 	"github.com/aws/aws-sdk-go/aws/session"
 	"github.com/aws/aws-sdk-go/service/sqs"
 	"github.com/prometheus/client_golang/prometheus"
+
 	log "github.com/sirupsen/logrus"
-	"golang.org/x/net/context"
 
 	"github.com/weaveworks/common/instrument"
 	"github.com/weaveworks/scope/app"

app/multitenant/user.go

@@ -4,7 +4,7 @@ import (
 	"fmt"
 	"net/http"
 
-	"golang.org/x/net/context"
+	"context"
 
 	"github.com/weaveworks/scope/app"
 )

app/pipe_router.go

@@ -6,8 +6,8 @@ import (
 	"sync"
 	"time"
 
+	"context"
 	log "github.com/sirupsen/logrus"
-	"golang.org/x/net/context"
 
 	"github.com/weaveworks/common/mtime"
 	"github.com/weaveworks/scope/common/xfer"

app/pipes.go

@@ -1,11 +1,11 @@
 package app
 
 import (
+	"context"
 	"net/http"
 
 	"github.com/gorilla/mux"
 	log "github.com/sirupsen/logrus"
-	"golang.org/x/net/context"
 
 	"github.com/weaveworks/scope/common/xfer"
 )

app/pipes_internal_test.go

@@ -11,9 +11,9 @@ import (
 	"testing"
 	"time"
 
+	"context"
 	"github.com/gorilla/mux"
 	"github.com/gorilla/websocket"
-	"golang.org/x/net/context"
 
 	"github.com/weaveworks/common/mtime"
 	"github.com/weaveworks/scope/common/xfer"

app/router.go

@@ -11,11 +11,11 @@ import (
 	"sync"
 	"time"
 
+	"context"
 	"github.com/PuerkitoBio/ghost/handlers"
 	"github.com/gorilla/mux"
 	log "github.com/sirupsen/logrus"
 	"github.com/ugorji/go/codec"
-	"golang.org/x/net/context"
 
 	"github.com/weaveworks/scope/common/hostname"
 	"github.com/weaveworks/scope/common/xfer"

app/router_test.go

@@ -9,9 +9,9 @@ import (
 	"testing"
 	"time"
 
+	"context"
 	"github.com/gorilla/mux"
 	"github.com/ugorji/go/codec"
-	"golang.org/x/net/context"
 
 	"github.com/weaveworks/common/test"
 	"github.com/weaveworks/scope/app"