Uses DB.getDataSource instead of constructing one from config values.

The reason for the change is that DB.createDataSource does some trick on "url" for specific DB drivers, and I want to use that.

On Heroku, the database URL should be passed as a command line parameter like the following:

```
web: target/universal/stage/bin/projectname -Dhttp.port=${PORT} -Ddb.default.driver=org.postgresql.Driver -Ddb.default.url=${DATABASE_URL}
```

And the DATABASE_URL starts with `postgres://`, not `jdbc:postgresql://`