Add missing requirements.txt

[sunpoet]

Fixes #266

Checklist

General Contributing

• [ ] Have you read the Code of Conduct and signed the CLA?

Is This a Code Change?

• [x] Non-code related change (markdown/git settings etc)
• [ ] Code Change
• [ ] Example/Test Code Change

Validation

• [ ] Have you ran tox?
• [ ] Do the tests pass?

[CLAassistant]

Thank you for your submission! We really appreciate it. Like many open source projects, we ask that you all sign our Contributor License Agreement before we can accept your contribution.
0 out of 2 committers have signed the CLA.

:x: sunpoet
:x: rhui-dbx
_{You have signed the CLA already but the status is still pending? Let us recheck it.}

[sunpoet]

The CLA does not work at all. If you click "Contributor License Agreement", it just open a new page with the same URL and content.

[greg-db]

Thanks for submitting this! I'll ask the team to review this.

Also thanks for letting us know about the issue with that CLA page. I'll ask the team to fix that up.

[rhui-dbx]

I couldn't get this working, so instead I duplicated the changes and merged them: https://github.com/dropbox/stone/pull/352

Closing this PR. Thanks!