Fix .travis.yml file

[SamGuay]

I am not familiar with the Travis CI steps and I feel stupid to not be able the steps to make it work.

Here's the instruction to build and deploy the book on the gh-pages branch automatically from the master branch: https://bookdown.org/yihui/bookdown/github.html.

I'm sure it's simple once you know how to do it.

[jcolomb]

This function is used to create the book only on the server side, not locally.
(the idea is that you do not have to push the changes in the html/pdf files, only push the change in the .md files and the rest is done on the server side (travis in this case).)

I think it is not very urgent to take care of, maybe worth looking into it when we get a pdf output ?