Make ginga compatible with latest astropy.io.fits/pyfits

Sometime in the last couple weeks, ginga became broken when use with the latest astropy.io.fits - I couldn't load any fits files due to an exception with an `ascardlist` function.

The reason is the use of the `Header.ascardlist` method.  Apparently that's been deprecated for some time, and is replaces by the `cards` property. See https://pyfits.readthedocs.org/en/latest/appendix/header_transition.html for more on this topic.

This small change fixes the problem, and makes ginga work again with the latest astropy.io.fits