Increase minimum required astropy version to 0.4
Add numpy and scipy as dependencies

Closes #141
Change tests to use stable astropy now that 0.4 is out
Remove unnecessary development build from travis

Now that astropy 0.4 is out the coverage build no longer needs to use the development version.
Merge pull request #143 from mwcraig/setup-py-update

This also ended up changing the travis setup to use stable astropy wherever possible