Include requirement.txt in sdist

[jonringer]

closes #7

requirement{,-tests}.txt are now included.

$ python setup.py sdist
....
copying requirements-test.txt -> ml_collections-0.1.0
copying requirements.txt -> ml_collections-0.1.0
...
creating dist
Creating tar archive
removing 'ml_collections-0.1.0' (and everything under it)

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Thanks for your pull request. It looks like this may be your first contribution to a Google open source project (if not, look below for help). Before we can look at your pull request, you'll need to sign a Contributor License Agreement (CLA).

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[jonringer]

@googlebot I signed it!

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