Lock rake ~> 0.8.7; rake 0.9 breaks Rails

So deploying to Heroku broke. This fixes it.

* http://stackoverflow.com/questions/6085610/ruby-on-rails-and-rake-problems-uninitialized-constant-rakedsl
* https://twitter.com/dhh/status/71966528744071169
Merge pull request #36 from SmartReceipt/rake_version

Lock rake ~> 0.8.7; rake 0.9 breaks Rails