Fix for py 3

py 3 install gives an error on import:

````
/usr/local/lib/python3.4/dist-packages/ggplot/components/palettes.py in <module>()
      9 from six.moves import range
     10 
---> 11 import husl
     12 from .xkcd_rgb import xkcd_rgb
     13 


ImportError: No module named 'husl'
````