use define_model_callbacks instead of define_callbacks

ActiveSupport::Callbacks.define_callbacks doesn't define the class
macros to create callbacks (anymore?). This means that in 2.0.0,
before_move, after_move, and around_move are missing.

ActiveModel::Callbacks.define_model_callbacks handles defining the
macros, and as a bonus adds the `:terminator` option itself.

lib/awesome_nested_set/awesome_nested_set.rb

@@ -88,7 +88,7 @@ def #{column}=(x)
         scope :roots, where(parent_column_name => nil).order(quoted_left_column_name)
         scope :leaves, where("#{quoted_right_column_name} - #{quoted_left_column_name} = 1").order(quoted_left_column_name)
 
-        define_callbacks :move, :terminator => "result == false"
+        define_model_callbacks :move
       end
 
       module Model
@@ -561,4 +561,4 @@ def quoted_scope_column_names
 
     end
   end
-end
+end