Remove bundled viridis colormap

[shoyer]

We currently bundle the viridis colormap with xarray:

xarray/xarray/plot/utils.py

Lines 15 to 26 in 1c37d9c

	def _load_default_cmap(fname='default_colormap.csv'):
	"""
	Returns viridis color map
	"""
	from matplotlib.colors import LinearSegmentedColormap
	# Not sure what the first arg here should be
	f = pkg_resources.resource_stream(__name__, fname)
	cm_data = pd.read_csv(f, header=None).values
	f.close()
	return LinearSegmentedColormap.from_list('viridis', cm_data)

https://github.com/pydata/xarray/blob/master/xarray/plot/default_colormap.csv

This is unnecessary these days and adds a small amount of packaging complexity. It's not unreasonable to expect at least matplotlib 1.5.x, which comes with viridis installed.

[jhamman]

+1. Do you want me to add this to my minimum versions update (#2204)?

[shoyer]

@jhamman sure, that sounds great!

[jhamman]

Done in be637d7.

[jhamman]

closed via #2204