Add pypy as an allowed failure
Possible fix for pypy

Idea from http://www.tismer.com/pypy/irc-logs/pypy/pypy.2013-11-18.log.html
Possible fix for pypy

Idea from https://github.com/cpbotha/nvpy/issues/19#issuecomment-9732016
Forgot sudo
Merge pull request #571 from hugovk/master

Add PyPy to the Travis build