Force upgrade of virtualenv on Travis.
Force upgrade of virtualenv on Travis.

brandm/live-py-plugin,donkirkby/live-py-plugin,donkirkby/live-py-plugin,brandm/live-py-plugin,donkirkby/live-py-plugin,brandm/live-py-plugin,donkirkby/live-py-plugin,donkirkby/live-py-plugin,brandm/live-py-plugin,brandm/live-py-plugin