Upgrade golang protobuf api to v2

[ilaleksin]

Hey,

gnostic-grpc has a few references to 'github.com/golang/protobuf' (api v1) in the generator package and in the end-to-end test example.

API v1 is deprecated and it is recommended to use 'google.golang.org/protobuf/proto' (api v2) instead.

What do you think about switching to protobuf api v2?

I would be glad to help with this upgrade so I have prepared PR #97 with the required changes.

Please review it and share your thoughts about these changes 😉