disable single table inheritance on roles

In case plugins have created the column for ineheritance, we do not want to mess it with our migration

db/migrate/20160907113604_normalize_permissions.rb

@@ -31,6 +31,8 @@ class NormalizePermissions < ActiveRecord::Migration[5.0]
   class Role < ActiveRecord::Base
     self.table_name = :roles
 
+    self.inheritance_column = :_type_disabled
+
     serialize :permissions, Array
   end