Remove heroku gem dependency

Since the `heroku` gem [is now deprecated](https://github.com/heroku/heroku/blob/726da666f4aca330c219bb12876cf8b32d79cc05/heroku.gemspec#L15-L19), we should no longer require it.

Users are expected to have the [Heroku Toolbelt](https://toolbelt.heroku.com/) installed.