Replace uuid dependency with internal library

Signed-off-by: Stephen J Day <stephen.day@docker.com>

context/trace.go

@@ -4,7 +4,7 @@ import (
 	"runtime"
 	"time"
 
-	"code.google.com/p/go-uuid/uuid"
+	"github.com/docker/distribution/uuid"
 )
 
 // WithTrace allocates a traced timing span in a new context. This allows a
@@ -45,7 +45,7 @@ func WithTrace(ctx Context) (Context, func(format string, a ...interface{})) {
 	f := runtime.FuncForPC(pc)
 	ctx = &traced{
 		Context: ctx,
-		id:      uuid.New(),
+		id:      uuid.Generate().String(),
 		start:   time.Now(),
 		parent:  GetStringValue(ctx, "trace.id"),
 		fnname:  f.Name(),