Restrict protobuf version to v1.2.0 (#49)

protobuf recently introduced a change in master that makes it
incompatible with the latest release. To prevent breakage, use latest
released version of protobuf. For more info, see
golang/protobuf#763 (comment)

.travis.yml

@@ -51,7 +51,9 @@ jobs:
         - popd
       script:
         - go get -u google.golang.org/grpc
-        - go get -u github.com/golang/protobuf/protoc-gen-go
+        - go get -d -u github.com/golang/protobuf/protoc-gen-go
+        - git -C $GOPATH/src/github.com/golang/protobuf checkout v1.2.0
+        - go install github.com/golang/protobuf/protoc-gen-go
         - mv $GOPATH/bin build/bin
       after_failure:
         - tree build