Use wheels for travis testing

Care is taken to use a numpy wheel whose version matches the scipy/astropy wheels.

Previous commit messages:
Use development astropy for test --coverage
Install coverage when needed
Add cython to the travis build environment, controlled by an environmental variable
Also added a few more words of advice to the travis part of the README
Restore -w option to build_sphinx
Change URL for numpy-specific wheels and make scipy an optional install
Fix latest stable numpy version
Several tweaks to improve performance and reduce unnecessary tests
Remove debugging statements and cython install in egg_only