Viridis exists but not in plt.cm.datad.keys()

Matplotlib version is 1.5.1 from Anaconda.

``` python
>>> import matplotlib.pyplot as plt
>>> plt.get_cmap('viridis')
<matplotlib.colors.ListedColormap at ...>
>>> names = plt.cm.datad.keys()
>>> 'viridis' in names
False
```

I don't understand why Viridis colormap is in the software but it is not part of colormap names in `plt.cm.datad` keys. Is this intentional or a bug?
