Switch to github.com/golang/dep for vendoring

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

vendor/github.com/opencontainers/image-spec/img/media-types.dot

@@ -0,0 +1,12 @@
+digraph G {
+  {
+    manifestList [shape=note, label="Manifest list\n<<optional>>\napplication/vnd.oci.image.manifest.list.v1+json"]
+    manifest [shape=note, label="Image manifest\napplication/vnd.oci.image.manifest.v1+json"]
+    config [shape=note, label="Image JSON\napplication/vnd.oci.image.config.v1+json"]
+    layer [shape=note, label="Layer tar archive\napplication/vnd.oci.image.layer.v1.tar\napplication/vnd.oci.image.layer.v1.tar+gzip\napplication/vnd.oci.image.layer.nondistributable.v1.tar\napplication/vnd.oci.image.layer.nondistributable.v1.tar+gzip"]
+  }
+
+  manifestList -> manifest [label="1..*"]
+  manifest -> config [label="1..1"]
+  manifest -> layer [label="1..*"]
+}