Handle when Celluloid.cores is nil

* It is possible for Celluloid.cores to return nil instead of an integer on some platforms. This calls to_i which will turn nil into 0 which will cause the worker pool size to be 2. I'm not sure if this is the best solution and I've already made a better fix for Celluloid in the case where I was running into this.
Merge pull request #1169 from douglaswth/handle_nil_celluloid_cores

Handle when Celluloid.cores is nil