Revive www/rubygem-turbolinks as www/rubygem-turbolinks-classic.

turbolinks was completely rewritten and released as version 5.x. The old
branch 2.5.x was renamed to turbolinks-classic.

www/gitlab still depends on the old version of rubygem-turbolinks.
Therefore its revived as a new port, while the updated turbolinks
port will satisfy www/rubygem-rails4.

PR: 213218
Approved by: swills (mentor)