PruneDeadWorkers > Resque_Worker always needs a logger, so give it a NullLogger

This morning Resque was failing and I had to restart some workers. After that, I couldn't get into the Dashboard because of this error:

```
PHP Fatal error:  Call to a member function log() on a non-object in /home/domains/domain.nl/app/releases/20140409085135/vendor/chrisboulton/php-resque/lib/Resque/Worker.php on line 440
```

This fixes it.