`no_update` is not applied to subclasses

[YoranBrondsema]

When you have something like

class TodoItem < ActiveRecord::Base 
  acts_as_list 
end 

class SubTodoItem < TodoItem; end

and do

item = SubTodoItem.create!
TodoItem.acts_as_list_no_update do
  item.update!(position: 123)
end

then the computation of the position for item is still enabled.

[swanandp]

Fixed through #315