Implemented fixes for compatibility with latest versions of pywcs and as...

Fixed a few compatibility issues with latest versions of pywcs (v1.11) and astropy (v0.4).  I replaced the calls to wcs_pix2sky/wcs_sky2pix with wcs_pix2world/wcs_world2pix.  I also added a patch to astropy_helper to redirect the astropy coordinate conversion methods to the equivalent methods in pywcs.