Print scikit-image version in test log

Currently `python setup.py test` prints the availability and version of packages used by Astropy to the console:
```
Numpy: 1.8.1
Scipy: 0.14.0
Matplotlib: not available
h5py: not available
```
@bsipocz For `photutils` it would be nice if this included `scikit-image` ... could you please try and figure out what needs to be done to get this printed?
(It will help with user bug reports we receive in the future.)