message: make gRPC use proto v3.6.0 instead of v3.5.0
diff: @@ -16,6 +16,12 @@ http_archive(
  url = "https://github.com/GoogleCloudPlatform/google-cloud-cpp/archive/v0.2.0.zip",
  )
  +http_archive(
  + name = "com_google_protobuf",
  + strip_prefix = "protobuf-3.6.0",
  + url = "https://github.com/google/protobuf/archive/v3.6.0.tar.gz",
  +)
  +
  new_http_archive(
  name = "com_google_benchmark",
  build_file = "cc/benchmark.BUILD",
  