The goal of this change is to inspect how Python 3 compatibility plays
in the Travis-CI environment, while not blocking the supported Python
2 builds.

IIUC, when using "system_site_packages", we're stuck with the Python
version on Ubuntu Trusty (2.7.6 and 3.4.3).  That's the reason behind
the specific "3.4" version added there.

[1] https://docs.travis-ci.com/user/languages/python/#Travis-CI-Uses-Isolated-virtualenvsSigned-off-by: NCleber Rosa <crosa@redhat.com>