Re-add DATABASE_URL

Maybe you can enlighten me on the full context of https://github.com/k2nr/dokku-mysql-plugin/commit/1c179fab6afbc983a851efb6168bb3b504c75442 but Heroku's buildpack depends on `DATABASE_URL` (see: https://github.com/heroku/heroku-buildpack-ruby/blob/fbc7046672abe06bf4774c451f4a029aecf1a12e/lib/language_pack/ruby.rb#L582-L586)

This PR re-adds `DATABASE_URL` but also leaves the other variables you added.

@k2nr thoughts?
Argh, `mysql` probably isn't good for everyone since the buildpack uses this to set the `adapter` which can be `mysql` or `mysql2` depending on what gem you use..