Consider updating to a newer version of github.com/golang/protobuf module

[dmitshur]

The current latest release of this google.golang.org/appengine module requires version v1.2.0 of the github.com/golang/protobuf module.

PR golang/protobuf#808 was merged in protobuf recently, removing its requirement on the largely-unused google.golang.org/genproto module. It's likely a new tagged release of protobuf will follow soon (/cc @dsnet).

This issue is for appengine module to consider updating its required version of protobuf module from v1.2.0 to the upcoming newer version. That will reduce the list of dependencies of the appengine module, and in turn anyone that requires this module. Thanks!