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.

probe/endpoint/connection_tracker.go

@@ -4,7 +4,7 @@ import (
 	"strconv"
 	"time"
 
-	log "github.com/Sirupsen/logrus"
+	log "github.com/sirupsen/logrus"
 	"github.com/weaveworks/scope/probe/endpoint/procspy"
 	"github.com/weaveworks/scope/probe/process"
 	"github.com/weaveworks/scope/report"

probe/endpoint/conntrack.go

@@ -13,7 +13,7 @@ import (
 	"time"
 	"unicode"
 
-	log "github.com/Sirupsen/logrus"
+	log "github.com/sirupsen/logrus"
 
 	"github.com/weaveworks/common/exec"
 )

probe/endpoint/dns_snooper_linux_amd64.go

@@ -8,11 +8,11 @@ import (
 	"sync"
 	"time"
 
-	log "github.com/Sirupsen/logrus"
 	"github.com/bluele/gcache"
 	"github.com/google/gopacket"
 	"github.com/google/gopacket/layers"
 	"github.com/google/gopacket/pcap"
+	log "github.com/sirupsen/logrus"
 )
 
 const (

probe/endpoint/ebpf.go

@@ -11,7 +11,7 @@ import (
 	"sync"
 	"syscall"
 
-	log "github.com/Sirupsen/logrus"
+	log "github.com/sirupsen/logrus"
 	"github.com/weaveworks/common/fs"
 	"github.com/weaveworks/scope/probe/endpoint/procspy"
 	"github.com/weaveworks/scope/probe/host"

probe/endpoint/procspy/proc_linux.go

@@ -10,8 +10,8 @@ import (
 	"syscall"
 	"time"
 
-	log "github.com/Sirupsen/logrus"
 	"github.com/armon/go-metrics"
+	log "github.com/sirupsen/logrus"
 
 	"github.com/weaveworks/common/fs"
 	"github.com/weaveworks/scope/common/marshal"

probe/endpoint/procspy/reader_linux.go

@@ -6,7 +6,7 @@ import (
 	"sync"
 	"time"
 
-	log "github.com/Sirupsen/logrus"
+	log "github.com/sirupsen/logrus"
 
 	"github.com/weaveworks/scope/probe/process"
 )