Lock the parent row when transitioning

This prevents deadlocks in some cases.
See #228 for more info.
This branch is only here to test the change.

lib/statesman/adapters/active_record.rb

@@ -70,7 +70,7 @@ def create_transition(from, to, metadata)
 
         transition = transitions_for_parent.build(transition_attributes)
 
-        ::ActiveRecord::Base.transaction do
+        @parent_model.with_lock do
           @observer.execute(:before, from, to, transition)
           unset_old_most_recent
           transition.save!