Travis-CI: drop support for Python 2.5

As announced at [1]: "Python 2.5 has been removed due to very low overall usage
and breaking changes in pip and virtualenv."

[1] http://about.travis-ci.org/blog/2013-11-18-upcoming-build-environment-updates/
Merge pull request #10 from certik/travis

Travis-CI: drop support for Python 2.5