WIP: Try NumPy instead of NumPyPy

[mgeier]

The PyPy versions available on Travis-CI are currently not documented, but I found a few that are working.

Note that these are currently pointing to version 5.8.x, which doesn't support NumPy yet:

pypy
pypy3

This is not working at all

pypy2.7

This is working, but we probably don't need it:

pypy3.5

The good news is that all tests for NumPy-related functions pass!

The bad new is that we are using numpy.frombuffer() in the tests for the buffer_read() method, and this doesn't work:

>       data = np.frombuffer(buf, dtype='float64').reshape(-1, 2)
E       AttributeError: '_cffi_backend.buffer' object has no attribute '__buffer__'

I'll try to find out more at a later time, but if somebody else knows something, please chime in!

[bastibe]

That error message is strange: I thought CFFI was developed by the PyPy guys. Why wouldn't they support the buffer protocol in their own implementation?