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.)
Wait ... shouldn't this be `KeyError` instead of `NameError`?

```python

In [1]: from collections import OrderedDict

In [2]: d = OrderedDict()

In [3]: d['asdf']
---------------------------------------------------------------------------
KeyError                                  Traceback (most recent call last)
<ipython-input-3-eee2588c77d3> in <module>()
----> 1 d['asdf']

KeyError: 'asdf'
```