Segmentation faults on PyPy on Travis

[banksJeremy]

I've had a couple of builds fail because of segfaults on PyPy on Travis.

It's possible that this is due to thread-unsafe behaviour that we're planning to remove, but it's doesn't seem to ever be an issue with CPython so I'm going to just switch back to using CPython on Travis for now. We can trying switching back to PyPy once we address potential concurrency issues.