Fixed sorting in activeadmin 1.0

This fixes Sorting on Active Admin 1.0.0.pre and Rails 4.0.4

I can't run the spec suite within activeadmin-mongoid, but they do pass when I run from my rails app,
 i.e. copy spec/unit/order_clause_spec.rb to railsapp/spec/models/

The spec suite needs to be fixed here.