Changed the default database file location

Changed the database file from `db/sample.yml` to `database.yml` to prevent errors when the `db` folder does not exist:

    bundle exec ruby yaml.rb 
    .../rom-demo-master/lib/yaml_adapter.rb:55:in `initialize': No such file or directory @ rb_sysopen - db/sample.yml (Errno::ENOENT)
    from .../rom-demo-master/lib/yaml_adapter.rb:55:in `open'