Apply rouge to Travis

Created by: juthilo

Rouge isn't a default dependency of the jekyll gem, so we need to tell Travis/S3 to manually install it before the docs validation etc can be run, right?

X-Ref: https://github.com/twbs/bootstrap/commit/bb939f0004ce96de8598c72197d39809f7af175c Example of failed test: https://travis-ci.org/twbs/bootstrap/jobs/29403415#L127

To: @mdo @cvrebert