\case deprecated use \begin\end{case}

Hi ! 

I'm working on nbconvert pdf export. 
I got the warning/error that \case was deprecated, to replace with \begin/\end{case}

Just in case you wish to check that the html is unchanged : [nbviewer link](http://nbviewer.ipython.org/urls/raw.github.com/Carreau/Probabilistic-Programming-and-Bayesian-Methods-for-Hackers/fix-case/Chapter1_Introduction/Chapter1_Introduction.ipynb)

With this patch I was able to compile [this pdf](http://elacave.lmdb.eu/~carreau/Chapter1_Introduction.pdf) from the first chapter.... This is still work in progress :-)