Update travis doc build to use wheels for sphinx and matplotib

This updates ``.travis.yml`` and ``.travis.pip.wheels`` to use wheels for the installation of matplotlib and sphinx. This is only relevant to the travis job that does the documentation.

The main reason for this is that we've been seeing a lot of random network problems when downloading matplotlib, so by using the wheels there are now two possible places to get matplotlib (the wheels server, *or* the matplotlib source distribution, if the wheel is unavailable).

This also involved tweaking the bash script that gets the wheels, because sphinx is pure-python, and there's a different wheel naming convnention there.