Replace PIL with Pillow

A [recent build failed](https://travis-ci.org/codeforamerica/bizfriendly-api/builds/18432445) on `pip install PIL`. Pillow is a fork of PIL that attempts to [address PIL’s install problems](https://pypi.python.org/pypi/Pillow/1.7.8#introduction), and might help Travis pass a test.

This branch is not tested.