Travis setup

OK, the fact that the doc already contains some broken links and ReadTheDocs does not complain when building the HTML (the warning is displayed when looking at the build logs but the build is still successsful) convinced me it is already a good to enable Travis.

so here it is. Closes #8

Note: the hook is not yet enabled in the Behat repo. I enabled Travis in my fork to test the config, and it is now green.