higher image test tolerances for matplotlib versions != 1.3.1

I would propose to give any mpl versions that do not match the version the baseline images were created with a high tolerance. We so often see failing image tests on non 1.3.1 mpl versions that show diff images with slightly shifted axes and labels and texts...

I think rigorous testing on mpl 1.3.1 is enough, other versions get higher tolerance but we still see if the plot can be executed with the respective mpl API.