Updated dependency to matplotlib v1.3.1

This PR updated the cartopy matplotlib dependency to v1.3.0. The tests no longer pass for matplotlib v1.2.0rc3, but the code still executes in exactly the same way. Cartopy v0.10.x will be the last series which guarantees this backwards compatibility, and indeed many of the shims needed within cartopy will be removed in future cartopy versions.

I've also added an inline HTML diff viewer so that we can see any problems which are occuring on travis, but not on local machines.

The primary difference has been as a result of the change in default for snapping of Collections (including pcolor).