replace deprecated matplotlib.nxutils

Update to `matplotlib` [version 1.3.0+](http://matplotlib.org/1.3.0/api/api_changes.html) where `matplotlib.nxutils` [doesn't exist any more](http://matplotlib.org/1.2.1/api/nxutils_api.html), discussed also [here](http://stackoverflow.com/questions/13375957/python-running-matplotlib-nxutils-points-inside-poly-on-mac/13386468#13386468).