vendor: Update github.com/gogo/protobuf, keeping all files

gvt fetch -a, because we need the protobuf files etc for regeneration

vendor/github.com/gogo/protobuf/GOLANG_CONTRIBUTORS

@@ -0,0 +1,5 @@
+The contributors to the Go protobuf repository:
+
+# This source code was written by the Go contributors.
+# The master list of contributors is in the main Go distribution,
+# visible at http://tip.golang.org/CONTRIBUTORS.