Fix empty formatter after initialization

In this commit
```git
https://github.com/bdurand/lumberjack/commit/5a232eb237c63609e4d45a5e6e65d4079874f608
```
**@formatter** was changed to **@_formatter**, therefore **attr_reader: formatter** has stopped working.
Therefore after initialization formatter returned **nil** and **logger.formatter.add** isn't working.