Switch to github.com/golang/dep for vendoring

Signed-off-by: Mrunal Patel <mrunalp@gmail.com>

vendor/github.com/opencontainers/go-digest/doc.go

@@ -1,3 +1,17 @@
+// Copyright 2017 Docker, Inc.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
 // Package digest provides a generalized type to opaquely represent message
 // digests and their operations within the registry. The Digest type is
 // designed to serve as a flexible identifier in a content-addressable system.