Upgrade the Protobuf API version

[adzil]

The Protobuf API used in this project is already outdated and cannot be used for Protobuf definitions that are compiled using a recent version of protoc-gen-go which uses this package instead.

Is there any plan to upgrade this? I can create a PR for this as I already upgraded it in my fork although I understand that this may break backward compatibility with the existing mailgun/groupcache/v2 API.