Ensure that fonts are present on travis when building docs.

If we publish the documents build from Travis it makes sense to ensure that all fonts are available 
when building docs. 

This fixes the docs build by:

*  Add free fallback fonts to examples that uses Courier and Helvetica
*  Add [Felipa](https://www.google.com/fonts/specimen/Felipa) as an alternative cursive font. None
of the current cursive fonts are free as far as I can see.
*  Install ```Humor Sans``` and ```Felipa``` on Travis. 

The results are available at http://jenshnielsen.github.io/mplddocs/examples/pylab_examples/text_handles.html, http://jenshnielsen.github.io/mplddocs/examples/pylab_examples/fonts_demo.htm, http://jenshnielsen.github.io/mplddocs/examples/showcase/xkcd.html and http://jenshnielsen.github.io/mplddocs/examples/pylab_examples/logo.html