Pin pants support down to python2.7 - dropping 2.6.

Testing the python3.3+ waters will happen seperately - this just drops support for running pants against 2.6 as agreed.

Testing Done:
CI away here: https://travis-ci.org/pantsbuild/pants/builds/40110691

Bugs closed: 722, 767

Reviewed at https://rbcommons.com/s/twitter/r/1278/