Use ruby-end in ruby-mode

`ruby-end` is currently a default package, but it's only enabled for Elixir mode and not for Ruby. :trollface: 

Really this is `ruby-end`'s fault because they have code to enable it by default but it isn't properly marked as autoload: https://github.com/rejeep/ruby-end.el/blob/master/ruby-end.el#L180