Fix code reloading in development mode

ActionDispatch::Callbacks.to_prepare reloads code on every requests, which is
the desired behaviour in development mode. The other version introduced by
b1d89ee only reloads code when Rails configures itself. Hence, when an
other plugin patches the same classes/modules, the patches introduced by
this plugin disappear so everything is broken in development mode.

Author: jbbarth

Diff for: init.rb

@@ -5,7 +5,7 @@
 require 'omniauth/dynamic_full_host'
 
 # Patches to existing classes/modules
-Rails.application.config.to_prepare do
+ActionDispatch::Callbacks.to_prepare do
   require_dependency 'redmine_omniauth_cas/account_helper_patch'
   require_dependency 'redmine_omniauth_cas/account_controller_patch'
 end