Use new heroku-api instead of old heroku gem

Hi, I like the ideas behind this gem, but found that I could not use the super old heroku gem.

This updates all of the heroku interaction to use the newer `heroku-api` gem.

Also, it sets up HEROKU_API_KEY instead of HEROKU_USER and HEROKU_PASS. The new Heroku::API suppors user and password, but I had some issues with it and didn't feel like making sure both worked, so this fork only supports the api key.


I'm not sure if you want to pull in these changes, but I thought I'd pull-request to give you the opportunity, and to help others find this branch should they need it.

I also did not update the version number, although I will in my master branch.