Improved compatibility with matplotlib 1.5
Matplotlib 1.5 introduces a bunch of new colormaps, and some other changes to setting colors. This PR removes all explicit references to the 'jet' colormap in favour of whatever the default is in the rcParams
structure ('jet' for mpl 1.x, 'viridis' for mpl 2.0).
This PR also fixes a quirk in the SegmentAxes.draw
meaning mpl 1.5 would draw blue boxes around tick labels.