travis: add automatic deployment of documentation
travis: add automatic deployment of documentation

As inspired by:
- http://www.steveklabnik.com/automatically_update_github_pages_with_travis_example
- https://github.com/boostorg/hana/blob/master/.travis.yml

dodheim/atria,AbletonAG/atria,Ableton/atria,dodheim/atria,AbletonAG/atria,dodheim/atria,AbletonAG/atria,Ableton/atria,Ableton/atria