fix for future nbviewer

Hi, 

I'm working on new version of nbviewer.
I saw that some example where where broken and not fully [up to date](http://gistpynb.herokuapp.com/urls/raw.github.com/jiffyclub/ipythonblocks/master/demos/learning_colors.ipynb).

It's still [not perfect](http://gistpynb.herokuapp.com/urls/raw.github.com/Carreau/ipythonblocks/acd6ffbf9f736b10c0b1428bbc34787c70d72fe0/demos/learning_colors.ipynb) but I'm working on it.