Cleanup Style Issues

```
$ bundle exec rake rubocop
Running RuboCop...
 39/39 files |============ 100 ===========>| Time: 00:00:01 

39 files inspected, no offenses detected
```
This is a good change but in the future I think `File.join(Director.put_path(self), 'backend', backend.name)` is even clearer.