Update matplotlib dependency to v1.3.1

WARNING! This PR changes >200 files and so may render your web browser (or computer!) inoperable!

Assuming it doesn't, this contains the updates necessary to make Iris compatible with matplotlib v1.3.1. Primarily this means changes to image files arising from the differences in the way mpl1.3.1 renders plots, but there are also a few code changes to account for differences introduced by mpl1.3.1.