Fix Issue#100

[genkioffice]

To fix KeyError in prettyplotlib/colors.py, I have modified following;

• Use axes.prop_cycle instead of axes.color_cycle since rcParams of matplotlib requires prop_cycle.

• Use cycler library to be consistent with the matplotlibrc official template. https://github.com/matplotlib/matplotlib/blob/master/matplotlibrc.template

• Use hex_colors instead of mpl_colors to be consistent with the matplotlibrc official template too.