WIP: *: update grpc-gateway dependencies

[gyuho]

To try grpc-ecosystem/grpc-gateway#82 (comment).

For #5865.

curl -L http://localhost:2379/v3alpha/kv/put -X POST -d '{"key": "Zm9v", "value": "YmFy"}'

curl -L http://localhost:2379/v3alpha/kv/range -X POST -d '{"key": "Zm9v"}'

curl -L http://localhost:2379/v3alpha/watch -X POST -d '{"request_union": {"create_request": {"key": "Zm9v"}}}'
# {"Error":"json: cannot unmarshal object into Go value of type etcdserverpb.isWatchRequest_RequestUnion","Code":2}

[gyuho]

Closing via grpc-ecosystem/grpc-gateway#82 (comment).

There's an incompatibility issue with gogoproto and golang/protobuf/jsonpb. Will file an issue to gogoproto.