Add support for Turbolinks 5
Add support for Turbolinks 5

Why:

* With Turbolinks 5 the event that's fired by Turbolinks when a page is
  loaded changed, therefore this gem does not support its latest
  version.

This change addresses the issue by:

* Refactoring the main script to add an event listener for the new event
  (`turbolinks:load`) on initialization.

davydotcom/retina_tag,davydotcom/retina_tag