fix not to require before using Celluloid actually

To suppress Celluloid's debug messages when celluloid is not used at all.
```
$ ruby -e 'require "celluloid"; 1;'
D, [2013-05-13T18:59:41.180011 #72519] DEBUG -- : Terminating 4 actors...
D, [2013-05-13T18:59:41.190204 #72519] DEBUG -- : Shutdown completed cleanly
```