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
Merge pull request #244 from pozorvlak/fix_rake

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