Revert "Only test with Python 3.3 on Travis"

This reverts commit c0d4e4b0f5499ac074b31ed1603404d415e02e91.

Looks like Travis only runs Ubuntu 12.04 LTS, the distributed Python
version of which is Python 3.2. If we insist on going with Python 3.3,
Travis will install its own version of Python, which cannot access
system packages like python3-pyqt4 or python3-gi.
Install python3-mock on Travis
Fallback to mock instead of unittest.mock
Merge pull request #31 from lewtds/fix-travis-mock

Fix unittest.mock on Travis