Use Astropy by default instead of PyFITS if possible

As @mindw mentioned in https://github.com/astropy/astropy/pull/1493, it is not currently possible to use Astropy instead of PyFITS for scikit-image. 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).