problem with ginga import

[KathleenLabrie]

In imexam/ginga_viewer.py, line 24 in commit b397573, there this line that fails:

wcsmod.use('AstropyWCS')

I believe that it should be:

wcsmod.use('astropy')

It does fix the import failure, but I have no idea if the rest of the code works. I just got fed up of seeing this error when I import imexam:

Ginga not installed, use other viewer, or no viewer

I have ginga installed and I'm not even using it and this error shows up all the time.

(Not submitting a PR because as I say above, I don't know if it breaks things afterwards, I just know that it fixes the import error.)

[Universebenzene]

Same as #252