Update import statements to use sirupsen/logrus.

Commands run:
```
$ git grep -l Sirupsen | grep -v vendor | xargs sed -i 's:github.com/Sirupsen/logrus:github.com/sirupsen/logrus:g'
$ gofmt -s -w app
$ gofmt -s -w common
$ gofmt -s -w probe
$ gofmt -s -w prog
$ gofmt -s -w tools
```
N.B.: following this commit, build fails with:
```
vendor/github.com/docker/docker/pkg/archive/archive.go:20:2: cannot find package "github.com/Sirupsen/logrus" in any of:
	/go/src/github.com/weaveworks/scope/vendor/github.com/Sirupsen/logrus (vendor tree)
	/usr/local/go/src/github.com/Sirupsen/logrus (from $GOROOT)
	/go/src/github.com/Sirupsen/logrus (from $GOPATH)
```
Resolution in subsequent commit.

prog/app.go

@@ -12,9 +12,9 @@ import (
 	"strings"
 	"time"
 
-	log "github.com/Sirupsen/logrus"
 	"github.com/gorilla/mux"
 	"github.com/prometheus/client_golang/prometheus"
+	log "github.com/sirupsen/logrus"
 	"github.com/tylerb/graceful"
 
 	billing "github.com/weaveworks/billing-client"

prog/main.go

@@ -12,7 +12,7 @@ import (
 	"strings"
 	"time"
 
-	log "github.com/Sirupsen/logrus"
+	log "github.com/sirupsen/logrus"
 
 	billing "github.com/weaveworks/billing-client"
 	"github.com/weaveworks/scope/app"

prog/main_test.go

@@ -4,7 +4,7 @@ import (
 	"flag"
 	"testing"
 
-	"github.com/Sirupsen/logrus/hooks/test"
+	"github.com/sirupsen/logrus/hooks/test"
 	"github.com/stretchr/testify/assert"
 )
 

prog/probe.go

@@ -10,9 +10,9 @@ import (
 	"strconv"
 	"time"
 
-	log "github.com/Sirupsen/logrus"
 	"github.com/armon/go-metrics"
 	"github.com/prometheus/client_golang/prometheus"
+	log "github.com/sirupsen/logrus"
 
 	"github.com/weaveworks/common/network"
 	"github.com/weaveworks/common/sanitize"