Enable python coverage / coveralls in travis-ci runs.

Testing Done:
CI went green here: https://travis-ci.org/pantsbuild/pants/builds/35584970
Coverage looks good under ci now - 57% + sane paths on the console output.
Coveralls also looks good with 61.48% (not sure why it sums differently) and sane paths that display real coverage annotated files: https://coveralls.io/builds/1223782

Bugs closed: 369, 581

Reviewed at https://rbcommons.com/s/twitter/r/969/