Fix a position bug occuring when deleting elements by updating acts_as_list

When I delete a page element, I cannot re-add further ones because of a validation error: "position already taken", see [this screen capture](https://www.dropbox.com/s/8hc2qu6rt8rbegk/alchemy_cms_bug_acts_as_list.mov).

Updating acts_as_list solves this issue.