The build fails with clang-4.0 (and clang-3.9) because the version of protobuf included with the grpc version we are using has a bug. This is a big rabbit hole, we may need to fix protobuf first (file a bug, submit a PR), then change the version of protobuf that grpc has pinned (file a bug, submit a PR), and then change our version of grpc (this bug, a future PR).

I am not sure what this means for our plans for he build matrix in #19, it might mean that we do not support older versions of grpc (e.g. v1.6.x), or that we do not support using the particular compiler / grpc version combos that are broken.