I looked at the Travis logs. It's not just that the tests are failing, 
instead the whole system crashed entirely, so that the stack trace is 
not even reported.
I will experiment with that.
Probably, I'll split off the wcs_utils tests and put them in their own 
pull request.

Duck-typing: I went for duck-typing initially, because it (i) simplifies 
the code and (ii) would avoid the problem for a larger group of inputs, 
e.g. some user might use pandas arrays instead of astropy tables (and I 
don't know if those are derived from ndarrays).