Add support for Travis CI

[Muffo]

The script:

• Verifies that the the pdf can be generated without errors
• Publishes the latest version to Github pages

In order for this to work, Travis CI must be configured for this project. You will also need to generate an authentication token for Github so that Travis can publish the latest version of the pdf.

This page has all the instructions but I am happy to help if they are not clear :)

The link to the pdf has also been added to the README so that people can see a preview of the book without installing Tex Live on their system. It won't work until Travis has been configured, but in the meantime you can see a preview of the result here: https://muffo.github.io/openssl-book/openssl-book.pdf