Basically your protoc-gen-go doesn‘t match the version of grpc. So sync them both to the latest version and reinstall protoc-gen-go will resolve the issue:
go get -u github.com/golang/protobuf/
cd github.com/golang/protobuf/
make
go get -u github.com/grpc/grpc-go
++++++++++++++++++++++++++++++++++++
cd /opt/gopath/src/github.com/golang/protobuf/protoc-gen-go/
go build
which protoc-gen-go