Fix Rails 6 zeitwork autoloading deprecation
Fix Rails 6 zeitwork autoloading deprecation

Including the gem in a Rails 6 app results in the following error message:
> DEPRECATION WARNING: Initialization autoloaded the constants ActionText::ContentHelper and ActionText::TagHelper.
> 
> Being able to do this is deprecated. Autoloading during initialization is going
> to be an error condition in future versions of Rails.

This change uses ActiveSupport's onload hook in order to ensure that the materialize helpers are added to the *newly loaded* ActionController::Base, whenever a reload occurs.

Fixes #192
mkhairi/materialize-sass