Don't nil Celluloid.logger

Celluloid.logger is defined once Celluloid is required. Sidekiq setting the Celluloid.logger to `nil` ensures that it doesn't play nice with other gems that require Celluloid.logger to be available. [Listen](https://github.com/guard/listen) is one such gem. A related issue, and the one that prompted this PR, is https://github.com/mperham/sidekiq/issues/1858. I'm using Rails 4.1 and Ruby 2.0.0-p353

Though the initial reporter just hacked away the log calls from the Listen gem and others thought this was Listen's problem I maintain this is something Sidekiq is doing badly. 

I'd be interested to know the thoughts behind setting it to `nil` if my fix here isn't acceptable, it came about with https://github.com/mperham/sidekiq/commit/e38a3d785a76a4e5b5ffbbec87e579e466185e4c though the comments don't explain that particular line.