Summary:
Pull Request resolved: https://github.com/facebookresearch/CrypTen/pull/62

* use fixed version of `torch nightly` and onnx in circle ci `setup_env` with support for the new `enable_onnx_checker` (see D19782292)
* remove duplicated environment setup in `config.yml`
* update `CONTRIBUTING.md` with installation instructions for latest dev version

Thanks vshobha for helping debug hairy dropout onnx operators!

Reviewed By: vshobha

Differential Revision: D20006464

fbshipit-source-id: 2ffa82343b178bccd3d0838bdb585b9f7b984618