Ensure that fonts are present on travis when building docs.

[jenshnielsen]

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 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

[tacaswell]

This does raise the question if the main docs are screwed up because I built them on my system which may not have all of the right fonts.

Compare http://matplotlib.org/examples/pylab_examples/fonts_demo.html and http://jenshnielsen.github.io/mplddocs/examples/pylab_examples/fonts_demo.html I am not sure where the changes in text position came from..

[jenshnielsen]

@tacaswell I updated the positions and relaxed the font requirements in #3801.
Might be worth back porting to 1.4.3 ?

It looks like the only issue with the main docs is a missing Cursive font. Prior to #3801 the only working Cursive font was Script MT which was hard coded into the example.

[pelson]

LGTM 👍