Workaround Pypy regression

There's a regression in the latest PyPy (2.4.0) which causes test failures (see #952) so allow it to fail.

Explicitly add the previous PyPy 2.3.1 so we still get useful results. 

(List of Travis CI's supported Python versions: https://github.com/travis-ci/travis-cookbooks/blob/master/ci_environment/python/attributes/default.rb )