The way this is composed, `database_url` will try to do `ENV[ENV['PGBACKUPS_DATABASE']]` if `ENV['PGBACKUPS_DATABASE']` is set.  To correct this, we could instead do:

    def database_url
      ENV["PGBACKUPS_DATABASE_URL"] || ENV["DATABASE_URL"]
    end

Which also addresses the problem with just one method.  Note that I'm using `PGBACKUPS_DATABASE_URL` instead of `PGBACKUPS_DATABASE` which I like better because it is more consistent with Heroku's `DATABASE_URL`.