Upgrade futures to pick up some fixes.

Testing Done:
None yet - twitter/commons ci is broken atm on a hard coverage dep.

I did find a safe looking changelog that seems to back up the patch version bump - no API breaks from the looks of things:
  https://code.google.com/p/pythonfutures/source/browse/CHANGES

2.1.6
=====

- Fixed a problem with files missing from the source distribution

2.1.5
=====

- Fixed Jython compatibility
- Added metadata for wheel support

2.1.4
=====

- Ported the library again from Python 3.2.5 to get the latest bug fixes

2.1.3
=====

- Fixed race condition in wait(return_when=ALL_COMPLETED)
  (http://bugs.python.org/issue14406) -- thanks Ralf Schmitt
- Added missing setUp() methods to several test classes

Bugs closed: 300

Reviewed at https://rbcommons.com/s/twitter/r/534/