Fix: cannot find package github.com/Sirupsen/logrus in dependencies.

Fix:
```
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)
```
via:
```
$ git grep -l Sirupsen | xargs sed -i 's:github.com/Sirupsen/logrus:github.com/sirupsen/logrus:g'
```

vendor/github.com/Azure/go-ansiterm/parser.go

@@ -5,7 +5,7 @@ import (
 	"io/ioutil"
 	"os"
 
-	"github.com/Sirupsen/logrus"
+	"github.com/sirupsen/logrus"
 )
 
 var logger *logrus.Logger

vendor/github.com/Azure/go-ansiterm/winterm/win_event_handler.go

@@ -9,7 +9,7 @@ import (
 	"strconv"
 
 	"github.com/Azure/go-ansiterm"
-	"github.com/Sirupsen/logrus"
+	"github.com/sirupsen/logrus"
 )
 
 var logger *logrus.Logger

vendor/github.com/docker/docker/pkg/archive/archive.go

@@ -17,7 +17,7 @@ import (
 	"strings"
 	"syscall"
 
-	"github.com/Sirupsen/logrus"
+	"github.com/sirupsen/logrus"
 	"github.com/docker/docker/pkg/fileutils"
 	"github.com/docker/docker/pkg/idtools"
 	"github.com/docker/docker/pkg/ioutils"

vendor/github.com/docker/docker/pkg/archive/changes.go

@@ -13,7 +13,7 @@ import (
 	"syscall"
 	"time"
 
-	"github.com/Sirupsen/logrus"
+	"github.com/sirupsen/logrus"
 	"github.com/docker/docker/pkg/idtools"
 	"github.com/docker/docker/pkg/pools"
 	"github.com/docker/docker/pkg/system"

vendor/github.com/docker/docker/pkg/archive/copy.go

@@ -9,7 +9,7 @@ import (
 	"path/filepath"
 	"strings"
 
-	"github.com/Sirupsen/logrus"
+	"github.com/sirupsen/logrus"
 	"github.com/docker/docker/pkg/system"
 )
 

vendor/github.com/docker/docker/pkg/archive/diff.go

@@ -10,7 +10,7 @@ import (
 	"runtime"
 	"strings"
 
-	"github.com/Sirupsen/logrus"
+	"github.com/sirupsen/logrus"
 	"github.com/docker/docker/pkg/idtools"
 	"github.com/docker/docker/pkg/pools"
 	"github.com/docker/docker/pkg/system"

vendor/github.com/docker/docker/pkg/archive/example_changes.go

@@ -13,7 +13,7 @@ import (
 	"os"
 	"path"
 
-	"github.com/Sirupsen/logrus"
+	"github.com/sirupsen/logrus"
 	"github.com/docker/docker/pkg/archive"
 )
 

vendor/github.com/docker/docker/pkg/fileutils/fileutils.go

@@ -10,7 +10,7 @@ import (
 	"strings"
 	"text/scanner"
 
-	"github.com/Sirupsen/logrus"
+	"github.com/sirupsen/logrus"
 )
 
 // exclusion returns true if the specified pattern is an exclusion

vendor/github.com/docker/docker/pkg/fileutils/fileutils_unix.go

@@ -7,7 +7,7 @@ import (
 	"io/ioutil"
 	"os"
 
-	"github.com/Sirupsen/logrus"
+	"github.com/sirupsen/logrus"
 )
 
 // GetTotalUsedFds Returns the number of used File Descriptors by

vendor/github.com/docker/docker/pkg/stdcopy/stdcopy.go

@@ -8,7 +8,7 @@ import (
 	"io"
 	"sync"
 
-	"github.com/Sirupsen/logrus"
+	"github.com/sirupsen/logrus"
 )
 
 // StdType is the type of standard stream

vendor/github.com/docker/docker/pkg/system/syscall_windows.go

@@ -4,7 +4,7 @@ import (
 	"syscall"
 	"unsafe"
 
-	"github.com/Sirupsen/logrus"
+	"github.com/sirupsen/logrus"
 )
 
 var (

vendor/github.com/docker/docker/pkg/term/windows/windows.go

@@ -10,7 +10,7 @@ import (
 	"sync"
 
 	ansiterm "github.com/Azure/go-ansiterm"
-	"github.com/Sirupsen/logrus"
+	"github.com/sirupsen/logrus"
 )
 
 var logger *logrus.Logger

vendor/github.com/docker/libcontainer/cgroups/fs/stats_util_test.go

@@ -6,7 +6,7 @@ import (
 	"fmt"
 	"testing"
 
-	"github.com/Sirupsen/logrus"
+	"github.com/sirupsen/logrus"
 	"github.com/docker/libcontainer/cgroups"
 )
 

vendor/github.com/opencontainers/runc/libcontainer/cgroups/fs/stats_util_test.go

@@ -6,7 +6,7 @@ import (
 	"fmt"
 	"testing"
 
-	"github.com/Sirupsen/logrus"
+	"github.com/sirupsen/logrus"
 	"github.com/opencontainers/runc/libcontainer/cgroups"
 )
 

vendor/github.com/weaveworks/billing-client/client.go

@@ -7,7 +7,7 @@ import (
 	"sync"
 	"time"
 
-	log "github.com/Sirupsen/logrus"
+	log "github.com/sirupsen/logrus"
 	"github.com/fluent/fluent-logger-golang/fluent"
 	"github.com/prometheus/client_golang/prometheus"
 	"golang.org/x/net/context"

vendor/github.com/weaveworks/weave/common/logging.go

@@ -5,7 +5,7 @@ import (
 	"fmt"
 	"strings"
 
-	"github.com/Sirupsen/logrus"
+	"github.com/sirupsen/logrus"
 )
 
 type textFormatter struct {