Deploy gh-pages branch from travis

## [WiP] for task #34 

To safely deploy with couscus from a travis build we need to take this things into consideration:

- [ ] Make sure the deployment is running only for master.
- [ ] Make sure it is not a pull request.
- [ ] Make sure we deploy once per build. E.G. don't deploy for every PHP version tested :smile:
- [x] Decide what to use to deploy: [Private Key](http://stackoverflow.com/questions/23277391/pushing-to-github-from-travis-ci) or [Personal Access Token](http://stackoverflow.com/questions/18027115/committing-via-travis-ci-failing)  
    - More info on [Personal Access Token](https://medium.com/@nthgergo/publishing-gh-pages-with-travis-ci-53a8270e87db)

Feel free to add commits and keep working on this together.