Generate mobi versions of GITenberg books

[rdhyee]

Looking at whether we can mobi (and maybe pdf) versions of GITenberg books by using calibre inside of travis.

[rdhyee]

I am able to run ebook-convert (part of Calibre) to convert an epub to mobi and pdf with the following commands on Ubuntu 14.04 LTS:

sudo apt-get -y update
sudo apt-get -y install xvfb
sudo apt-get -y install calibre

# get Huck Finn epub
wget https://github.com/GITenberg/Adventures-of-Huckleberry-Finn_76/releases/download/0.1.7/Adventures-of-Huckleberry-Finn.epub

# mobi
ebook-convert Adventures-of-Huckleberry-Finn.epub Adventures-of-Huckleberry-Finn.mobi

#pdf -- need xvfb to run
xvfb-run ebook-convert Adventures-of-Huckleberry-Finn.epub Adventures-of-Huckleberry-Finn.pdf