Switch to mainline flanker

Summary:
Our fork for flanker is out of date and my previous bugs have been fixed by them in other commits. Our fork only had 3 changes.

This issue is closed and I validated the fix: https://github.com/mailgun/flanker/pull/36#issuecomment-50381678

Do we still need this? Can we configure our VM to use the proper DNS server elsewhere? https://github.com/inboxapp/flanker/commit/ecbb70a133324fb5e6ff06b9546456905c7537aa

And I believe this was fixed in some other flanker commit: https://github.com/inboxapp/flanker/commit/32524df59b2ff27fd3236073210bde750ae4b24b

At least one user has also complained about decoding errors that are probably likely to our fork being left behind. I couldn't replicate the error with the latest version on PyPI.

I'm not sure the best way to get this deployed since our `pip install` has been cloing our fork locally. Running setup.sh worked in my VM to overwrite our fork with the new version, but this might need some TLC during production rollout.

Test Plan: All tests should still pass.

Reviewers: spang, emfree

Reviewed By: emfree

Differential Revision: https://review.inboxapp.com/D661