Revert "Travis CI is based on Ubuntu 12.04"

I was mistaken: Ubuntu 12.04 comes with Python 2.7 by default, and
python-pygame is available for Python 2.7 only.

I've no idea why tox fails to pick it up.

This reverts commit 971c9a0ff1e2be9f5b6d9ec2b94b71566f751bf3.
Don't use tox for running tests