support protoc version < 3.2.0 (#594)
support protoc version < 3.2.0 (#594)

* support protoc version < 3.2.0

* add comment to old protobuf code block

* update according to code reviewer's comment

microsoft/onnxruntime,microsoft/onnxruntime,microsoft/onnxruntime,microsoft/onnxruntime,microsoft/onnxruntime,microsoft/onnxruntime,microsoft/onnxruntime,microsoft/onnxruntime,microsoft/onnxruntime,microsoft/onnxruntime,microsoft/onnxruntime,microsoft/onnxruntime