Improve gh-pages workflow from a CI perspective

[enriquecaballero]

It might be a good idea to improve the gh-pages workflow by matching those done by others via the use of deploy keys

References:

• https://djw8605.github.io/2017/02/08/deploying-docs-on-github-with-travisci/
• https://gist.github.com/domenic/ec8b0fc8ab45f39403dd
• http://stackoverflow.com/questions/23277391/how-to-publish-to-github-pages-from-travis-ci
• http://ricostacruz.com/cheatsheets/travis-gh-pages.html

[enriquecaballero]

v1.4.0 addresses some CI issues that might arise by allowing more customizations, but still leverages git subtree to push to gh-pages. Need to figure out more efficient way of pushing to gh-pages.