Remove Python 2.5 as Travis CI Py2.5 support ended

See http://blog.travis-ci.com/2013-11-18-upcoming-build-environment-updates/
and https://github.com/travis-ci/travis-ci/issues/1668#issuecomment-29151484

Current supported versions:

- 2.6
- 2.7
- 3.2
- 3.3
- 3.4
- pypy

(Source: http://docs.travis-ci.com/user/ci-environment/#Python-VM-images)