Fix issue with frozen constants in RailsAdmin at v1.0.0

Fixes #17

Author: rikkipitt

lib/rails_admin_history_rollback/extensions/paper_trail/auditing_adapter.rb

@@ -9,9 +9,6 @@ def version_id
           @version.id
         end
       end
-      class AuditingAdapter
-        COLUMN_MAPPING[:rollback] = :id
-      end
     end
   end
 end