Use wheels with TravisCI for NumPy, etc.

For NumPy and SciPy, we should consider using wheels (via pip) instead of building from source---which we don't actually do at all right now because it is so slow.

Something similar to what they are doing here, perhaps.
https://github.com/astropy/package-template/pull/44

This is just a placeholder. I can definitely tackle this at some point, just not immediately.