Updated astropy-helpers to latest version.

This version includes fixes for the coverage testing in affiliated packages.
Merge pull request #63 from astrofrog/update-astropy-helpers-2

Updated astropy-helpers to latest version.
Use wheels on travis

Care is taken to use a numpy wheel whose version matches the scipy/astropy wheels.
Use development astropy for test --coverage
Install coverage when needed

Lines to connect to coveralls.io are also included but need to be uncommented to work.
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
Increment numpy version to 1.8.1
Change URL for numpy-specific wheels and make scipy an optional install
Fix typo
Fix latest stable numpy version
Attempt to debug a travis problem with finding wheels
Revert "Attempt to debug a travis problem with finding wheels"

This reverts commit bcebdc425344970747b8d43d5d6417cfe33758d5.
Make pip install much more verbose for debugging
Remove trailing slash from URL
Add trailing slash to spoke URL