It varies between systems if /usr/bin/python is Python 2.x or Python
3.x. Since we now only work with Python 3.x we should be more explicit
in our scripts and documentation.
Travis are dropping older Python environments, so update things to
something that reflects what current distributions are using (e.g. Red
Hat Enterprise Linux 7).
This commit enables running the unit tests on Travis for Python 2.6,
2.7, 3.3, and 3.4. Note that Travis does not support Python 2.4, so
we cannot test there.
Pierre Ossman
UXabre
Solly Ross