Fix "uninitialized constant Rake::DSL" on Heroku.

From http://stackoverflow.com/questions/6181312/how-to-fix-the-uninitialized-constant-rakedsl-problem-on-heroku/6263256#6263256

Rakefile

@@ -2,6 +2,7 @@
 # Add your own tasks in files placed in lib/tasks ending in .rake,
 # for example lib/tasks/capistrano.rake, and they will automatically be available to Rake.
 
+require 'rake/dsl_definition'
 require File.expand_path('../config/application', __FILE__)
 
 Growstuff::Application.load_tasks