Enable Travis tests

Adds travis testing on Python 2.6 and 2.7. I had to grab [1] to get Python 2.6 to run without recursion errors, as in [2].

Passes! https://travis-ci.org/flacjacket/qtile/builds/28807261

[1] flacjacket@487466e49deb01867716a05ec36336c24e1a5085
[2] https://travis-ci.org/flacjacket/qtile/jobs/28804982