[travis] use Xenial for PyPy, don't allow PyPy2 failures
[travis] use Xenial for PyPy, don't allow PyPy2 failures

PyPy3 + numpy is currently broken see:

https://github.com/numpy/numpy/issues/13384

PyAV-Org/PyAV,PyAV-Org/PyAV,mikeboers/PyAV,mikeboers/PyAV