Cover PyPy and add PyPy 3

Travis CI has upgraded PyPy from 2.2.1 to 2.3.1, and have added PyPy 3. 

(They've also upgraded from 2.7.6 to 2.7.8 and 3.4.0 to 3.4.1.)

http://blog.travis-ci.com/2014-07-24-upcoming-build-environment-updates/

 * This means we can now cover PyPy 2 as the PyPy bug reported via https://github.com/python-pillow/Pillow/issues/640 is released in 2.3.1 (so closes #640).

Covering PyPy is still slow: about 6 minutes with, versus 1 minute without. Is this a problem? PyPy is built first and the other builds can run concurrently.

* We can also add PyPy 3 to the builds.