Create a direct grpc gem dependency in gitlab-ce and pin to 1.8.x

Due to problems in grpc 1.9.x we want to stick with grpc 1.8.x. #1029 (closed)

But we don't explicitly state this in gitlab-ce's Gemfile. We should add an explicit dependency on the 'grpc' gem and make sure it does not get accidentally upgraded beyond 1.8.x.