use Heroku API gem/key instead of username/password

Great gem; thank you very much for putting this together. I use it in a staging app where I don't need a worker process running all the time, just when I enqueue a job while testing. It's perfect. Thanks!

This pull request uses the ``heroku-api`` gem with your Heroku API key, instead of using the (depricated) ``heroku`` gem with your username/password.

You can find your Heroku API key by running ``heroku auth:token`` from your command line.

This change was inspired by @maxdunn210's [hirefire cedar stack patch on stackoverflow](http://stackoverflow.com/questions/9830629/rails-3-mailer-and-delayed-job-heroku).