www/ruby-actioncable51: add verson 5.1.5 package

Action Pack is a framework for handling and responding to web requests. It
provides mechanisms for *routing* (mapping request URLs to actions), defining
*controllers* that implement actions, and generating responses by rendering
*views*, which are templates of various formats. In short, Action Pack
provides the view and controller layers in the MVC paradigm.

This is for Ruby on Rails 5.1.

(taca)