dotenv instead of Rails credentials

The deployment is done on a Dokku server. It sets the DATABASE_URL
automatically.

config/database.yml

@@ -9,7 +9,7 @@ production:
   primary:
     <<: *default
     # database: storage/production.sqlite3
-    url: <%= Rails.application.credentials.db.url %>
+    url: <%= ENV['DATABASE_URL'] %>
     pool: 10
   cache:
     <<: *default