grpcio>=1.2.0
tenacity==4.10.0
protobuf==3.5.2.post1

grpcio>=1.2.0
tenacity==4.10.0
protobuf>=3.6.0

Change protobuf requirement to >=3.6.0
Change protobuf requirement to >=3.6.0

Change the minimum required version of protobuf to 3.6.0 to avoid
incompatibility issues of version 3.5.2.post1 with protobuf files
generated with newer versions.
