Conditionally stops checking if generated file are up-to-date

Golang changed the way of gzip compression between go1.6.1 and 1.6.1+.
So the gzipped proto.FileDescriptorProto always have diff.
Thus we cannot assume that generated files have the same contents as the
ones regenerated in Travis CI.

.travis.yml

@@ -19,7 +19,7 @@ install:
 - go get github.com/gengo/grpc-gateway/examples/server
 script:
 - make realclean && make examples
-- test -z "$(git status --porcelain)" || (git status; git diff; exit 1)
+- if ! go version | grep devel; then test -z "$(git status --porcelain)" || (git status; git diff; exit 1); fi
 - env GLOG_logtostderr=1 go test -race -v github.com/gengo/grpc-gateway/...
 - golint github.com/gengo/grpc-gateway/protoc-gen-grpc-gateway/...
 - golint github.com/gengo/grpc-gateway/runtime/...