new port: www/rubygem-mousetrap-rails

Mousetrap is a JavaScript library for handling keyboard shortcuts.
This gem integrates Mousetrap with the Rails asset pipeline.

WWW: http://kugaevsky.github.io/mousetrap-rails/

PR: 199918
Differential Revision: https://reviews.freebsd.org/D2501
Submitted by: Torsten Zuehlsdorff <ports@toco-domains.de>
Approved by: swills (mentor)