Remove Python 2.5 from .travis.yml

Looks like Travis CI dropped Python 2.5 support [recently](http://about.travis-ci.org/blog/2013-11-18-upcoming-build-environment-updates/). The supported versions can found here: http://about.travis-ci.org/docs/user/ci-environment/#Python-VM-images

The fact that we can't test Python 2.5 compatibility on Travis CI anymore is concerning though.