Bug 1272207 - Travis: Force virtualenv to use Python 2.7.11
Bug 1272207 - Travis: Force virtualenv to use Python 2.7.11

Since if not specified it defaults to `/usr/bin/python`, which on
Travis Precise containers is 2.7.6 and Trusty GCE is 2.7.3, since the
Python 2.7.11 install they provide is not system-installed but instead
just placed on the `PATH`.

This means that until now we've not actually been testing using the
same version of Python as has been running in production :-/

kapy2010/treeherder,tojon/treeherder,tojonmz/treeherder,edmorley/treeherder,tojonmz/treeherder,KWierso/treeherder,tojonmz/treeherder,akhileshpillai/treeherder,KWierso/treeherder,KWierso/treeherder,kapy2010/treeherder,glenn124f/treeherder,KWierso/treeherder,tojonmz/treeherder,akhileshpillai/treeherder,tojon/treeherder,edmorley/treeherder,glenn124f/treeherder,edmorley/treeherder,glenn124f/treeherder,edmorley/treeherder,kapy2010/treeherder,tojonmz/treeherder,kapy2010/treeherder,akhileshpillai/treeherder,akhileshpillai/treeherder,tojonmz/treeherder,kapy2010/treeherder,glenn124f/treeherder,akhileshpillai/treeherder,glenn124f/treeherder,glenn124f/treeherder,tojon/treeherder,tojon/treeherder,akhileshpillai/treeherder