Remove Python 2.5 from .travis.yml

Looks like Travis CI dropped Python 2.5 support recently. The supported versions can found here:
http://about.travis-ci.org/docs/user/ci-environment/#Python-VM-images
Leave a hint about python 2.5 support.

Just to be clear: As of now, buildbot still supports 2.5; it's just that we can't test that version with Travis CI anymore.
Merge pull request #978 from jpommerening/drop-python2.5-travis-env

Remove Python 2.5 from .travis.yml