Add Coveralls support

I noticed the coveralls badge in [django-widget-tweaks](https://github.com/kmike/django-widget-tweaks).  Seems like a code coverage badge probably wouldn't hurt.

I removed Python 2.5 support from Travis tests because coveralls can't be used otherwise.  The tox file doesn't test 2.5 anyways so support was basically already dropped previously.

I also think adding the `?branch=master` at the end of the Travis badge may fix the weird oddity we saw earlier with an old commit from a pushed tag messing up the build status.