Bump to latest go-digest.

This renames the license file (so automated tooling can find it, which I care
about) and also pulls in some documentation and comment changes, plus some
functional changes:

PR#33 -- future-proof the algorithm field.
PR#34 -- disallow upper case in hex portion.

No changes appear to be required to containerd code.

Signed-off-by: Ian Campbell <ian.campbell@docker.com>

vendor/github.com/opencontainers/go-digest/digester.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
 
 import "hash"