Add pages-gem to fix dependencies mismatch with github pages

I was having some build issues when pushing to gh-pages, but after following this https://help.github.com/articles/using-jekyll-with-pages#troubleshooting, it was fixed by using the [github-pages gem](https://github.com/github/pages-gem).

I guess is just a matter of enforcing some gem and dependencies versions. But for now it is solved with up-to-date gems.