Use unscoped blocks instead of chaining

Chaining unscoped with further scopes negates the unscoped call as per
the documentation: http://apidock.com/rails/ActiveRecord/Base/unscoped/class

Issue: https://github.com/swanandp/acts_as_list/issues/120
Hi Swanand, this is because moving a node into a new scope should add the node to the end of the list unless the position is explicitly specified in the update. Seems like the most reasonable expectation :)