Change plotting examples from pywcsgrid2 to wcsaxes

There's a few examples that use `pywcsgrid2` and the install instructions talk about it:
https://github.com/astropy/pyregion/search?q=pywcsgrid2&type=Code&utf8=%E2%9C%93

I guess this should be changed to [wcsaxes](http://wcsaxes.readthedocs.org/en/latest/) or [aplpy](http://aplpy.readthedocs.org/en/latest/)?
Filed a wcsaxes issue: https://github.com/astrofrog/wcsaxes/issues/135