Use new migration style in HABTM join table

Yes, we're on Rails 3.1 now.

activerecord/lib/active_record/associations.rb

@@ -1424,16 +1424,12 @@ module ActiveRecord
       # join table with a migration such as this:
       #
       #   class CreateDevelopersProjectsJoinTable < ActiveRecord::Migration
-      #     def self.up
+      #     def change
       #       create_table :developers_projects, :id => false do |t|
       #         t.integer :developer_id
       #         t.integer :project_id
       #       end
       #     end
-      #
-      #     def self.down
-      #       drop_table :developers_projects
-      #     end
       #   end
       #
       # Adds the following methods for retrieval and query: