Only define :version accessor for AR::Base subclasses that call has_paper_trail.

lib/paper_trail/has_paper_trail.rb

@@ -3,9 +3,6 @@ module PaperTrail
 
     def self.included(base)
       base.send :extend, ClassMethods
-
-      # The version this instance was reified from.
-      attr_accessor :version
     end
 
 
@@ -24,6 +21,9 @@ module PaperTrail
         # any more ActiveRecord models than we have to.
         send :include, InstanceMethods
 
+        # The version this instance was reified from.
+        attr_accessor :version
+
         cattr_accessor :ignore
         self.ignore = (options[:ignore] || []).map &:to_s