Use Astropy by default instead of PyFITS

As @mindw mentioned in https://github.com/astropy/astropy/pull/1493, it is not currently possible to use Astropy instead of PyFITS for veusz. PyFITS is going to be deprecated on a timescale of a ~year so it is now time to switch to using ``astropy.io.fits`` whenever possible. I wasn't sure whether it would make sense to have the fallback on PyFITS if Astropy is not installed or whether we should do a clean break and simply require Astropy? (I'm happy to update this PR).

I had issues building veusz (will open a separate issue) so cannot test that this works correctly at the moment - but maybe you make use of Travis?