Fix method redefined warning

This fixes the following warning:

  omniauth-radius-rails/lib/kracken/config.rb:14: warning: method redefined; discarding old user_class

It would be good to fix things to support customizing the user class for
long term flexibility, but it was changed due to some issues with Rails
autoloading retaining old class references. Instead we document it is
likely deprecated and that the reader returns the static constant now.

Author: cupakromer

lib/kracken/config.rb

@@ -1,8 +1,11 @@
 module Kracken
   class Config
-    attr_accessor :app_id, :app_secret, :user_class
+    attr_accessor :app_id, :app_secret
     attr_writer :provider_url
 
+    # @deprecated the associated reader returns static `::User`
+    attr_writer :user_class
+
     def initialize
       @user_class = nil
     end