Run diff-cover and diff-quality on Travis CI

PR mainly for demonstration.

This is currently failing but demonstrates how to reproduce https://github.com/edx/diff-cover/issues/94: "TypeError and SyntaxError with Python 3.x":

Failing 3.x builds:
https://travis-ci.org/hugovk/diff-cover/builds/29128516

If not exactly like this, it should at least somehow be part of the Travis CI build to ensure the commands can actually be run on Python 3.x. Either that, or more unit tests are needed to reproduce #94. Or better still, run the commands on Travis AND have more unit tests :)

I also added py32 to the envlist, but for some reason 3.2 isn't run on Travis.