text: Title: Use of matplotlib's colormap viridis
  Body: When I use matplotlibs new colormap _viridis_ and add a colorbar in a figure, matplotlib2tikz returns an AssertionError, because _viridis_ is not an linear-segemented-colormap.
  
  ``` python
  import numpy as np
  import matplotlib.pyplot as plt
  import matplotlib.cm as cm
  import matplotlib2tikz
  
  x, y = np.meshgrid(np.linspace(0, 1), np.linspace(0, 1))
  z = x**2 - y**2
  
  plt.pcolormesh(x, y, z, cmap=cm.viridis)
  plt.colorbar()
  
  matplotlib2tikz.save('test.tex')
  ```
  
  Is it possible to use pgfplots built-in colormap viridis for the colorbar specification with `colormap/viridis`, `point meta min=x` and `point meta max=y` in the axis options or is there a problem with different colormap-calculations/-interpolations?
  