Adding rails4 support

Rails 4 will call `call` (pun intended) on the `Formatter` objects so this will send it back to the `Logger` object. It will add the config logger (lumberjack) and then call `super` (subclass of `::Logger`) which will then call `format_message`, which is `formatter.call`. [Implementation here](http://apidock.com/ruby/Logger/format_message)