Update gh pages gem and fix encoding

This is my attempt at fixing GH-170.

Bumping github-pages from `8` to `18` is kinda risky and although it fixes my problem we should be very careful with that. Looking at https://github.com/github/pages-gem/releases it seems that we won't have much trouble with it, but it's always good to double check if nothing broke because of that ;-)

/cc @beatrizcp87 @thiagogabriel @maurogeorge