Passing App Argument to Rake Task

Passing the `:app` argument to the `rake rails_config:heroku` rake tasks requires that the argument is specified in the rake task. (See http://robots.thoughtbot.com/how-to-use-arguments-in-a-rake-task) I've added that here. Now, you can specify the app name when calling the rake task:
`bundle exec rake rails_config:heroku\[myapp\]`
This should probably also be added to the `README`, but that's your decision to make, so I didn't include it in this pull request.