Don't stomp on the revisiable_number setup work already done corectly.

The revisable_number is altered a second time in the revision_setup before_create callback, after already
being set up correctly in the revisable's to_revision method.  This results in duplicate revisable
numbers after the first revision, and incorrect numbers thereafter.

lib/acts_as_revisable/acts/revision.rb

@@ -72,7 +72,6 @@ def revision_setup #:nodoc:
         self[:revisable_is_current] = false
         self[:revisable_branched_from_id] = current_revision[:revisable_branched_from_id]
         self[:revisable_type] = current_revision[:type] || current_revision.class.name
-        self[:revisable_number] = (self.class.maximum(:revisable_number, :conditions => {:revisable_original_id => self[:revisable_original_id]}) || 0) + 1
       end
 
       def grab_my_branches