Switch to github.com/golang/dep for vendoring

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

vendor/github.com/containers/storage/docs/oci-storage-diff.md

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+## oci-storage-diff 1 "August 2016"
+
+## NAME
+oci-storage diff - Generate a layer diff
+
+## SYNOPSIS
+**oci-storage** **diff** [*options* [...]] *layerNameOrID*
+
+## DESCRIPTION
+Generates a layer diff representing the changes made in the specified layer.
+If the layer was populated using a layer diff, the result aims to be
+bit-for-bit identical with the one that was applied, including the type of
+compression which was applied.
+
+## OPTIONS
+**-f | --file** *file*
+
+Write the diff to the specified file instead of stdout.
+
+**-c | --gzip**
+
+Compress the diff using gzip compression.  If the layer was populated by a
+layer diff, and that layer diff was compressed, this will be done
+automatically.
+
+## EXAMPLE
+**oci-storage diff my-base-layer**
+
+## SEE ALSO
+oci-storage-applydiff(1)
+oci-storage-changes(1)
+oci-storage-diffsize(1)