Update WCS slicing to fix bug noted in astropy #2909

Astropy #2909 (https://github.com/astropy/astropy/pull/2909) noted an error in the treatment of WCS slices.  This PR pulls the astropy changes into spectral_cube.  After the 0.4.2 astropy release, we can change the version requirement of spectral_cube to astropy 0.4.2 and remove this entirely.