Parse Port from DATABASE_URL

Heroku Postgres uses the default port for its default database, but
Heroku production databases do not use the default port.

As specified in the note here:
https://devcenter.heroku.com/articles/upgrade-heroku-postgres-with-pgbackups#promote-new-database

> Please take care to note that the port number for your database may
> change during this process. You should ensure that the port is parsed
> correctly from the URL.