Fix ActiveRecord::Base namespacing
Fix ActiveRecord::Base namespacing

* AD::ActiveRecordStore was moved out from AR, so calling Base only
  doesn't work anymore.

maclover7/activerecord-session_store,sikachu/activerecord-session_store,maschwenk/activerecord-session_store,Ingelheim/activerecord-session_store,buntine/activerecord-session_store,gregmolnar/activerecord-session_store,Partyista/activerecord-session_store,rails/activerecord-session_store