Switch to Python 3.5
Switch to Python 3.5

It is the runtime we're expecting and `python3.4-venv` is not allowed on
Travis CI.

thomasleese/will-there-be-space,thomasleese/will-there-be-space,thomasleese/will-there-be-space,tomleese/will-there-be-space,thomasleese/will-there-be-space,thomasleese/will-there-be-space,tomleese/will-there-be-space,tomleese/will-there-be-space,tomleese/will-there-be-space,tomleese/will-there-be-space