Use DATABASE_URL envar even if there's no config

Without this change support of Rails 4.1 with sequel on Heroku is broken.
cf. https://discussion.heroku.com/t/rails-4-1-database-yml-no-longer-overwritten-on-heroku/550
Merge pull request #71 from dividedmind/support-no-database-yml

Use DATABASE_URL envar even if there's no config