Force Python 2.6 and 3.3 back to Trusty; add pypy testing

I have no idea how the main repo is successfully building on Travis now, but my fork is failing horribly because it can't install python 2.6 or 3.3 in the now-default ubuntu Xenial install and presumably even if edsu/pymarc is still defaulting to Trusty for some reason it will eventually get moved to default to Xenial. I can't even find a setting to set the default within Travis.