Use wheels on travis

Care is taken to use a numpy wheel whose version matches the scipy/astropy wheels.

This is an alternative to https://github.com/astropy/package-template/pull/37 that also addresses #24 and hopefully addresses the issue that @cdeil had with https://github.com/astropy/package-template/pull/37.

An example of the builds it produces on travis is at https://travis-ci.org/astropy/ccdproc/builds/21107695

There are at least few things that should be done before this is merged, if this seems like the way to go:

- [ ] Move the wheels on my server to either of the astropy wheelhouses (I'm willing to host, but adding a third place to look doesn't seem helpful).
- [ ] Address omission of ``-w`` in sphinx build
- [ ] Confirm what the minimum coverage expectations are for affiliated packages.

The script that produced these wheels on an Ubuntu VM isn't included in this PR, though I'm happy to share it.