Avoiding `grpcio==1.6.0` in deps. (#4096)
Avoiding `grpcio==1.6.0` in deps. (#4096)

This is due to `google-gax` doing the same, which has broken
RTD builds:

https://readthedocs.org/projects/google-cloud-python/builds/6063446/

The motivation for avoiding `grpcio==1.6.0` is:

https://github.com/grpc/grpc/issues/12455
googleapis/python-logging,googleapis/python-logging