support protobuf 4.0.0

[cruvie]

protobuf 4.0.0 is released, and grpc not capable with it

Because grpc 4.0.1 depends on protobuf >=2.0.0 <4.0.0 and ss_api depends on protobuf 4.0.0, grpc 4.0.1 is forbidden.
So, because ss_api depends on grpc 4.0.1, version solving failed.

[shadone]

I am hitting the same issue, please update the package to support protobuf 4.0.0

Here is a related issue in the protobuf package repo google/protobuf.dart#922

[mraleph]

I have released new version of grpc which should solve the issue.