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 (celluloid/celluloid/pull/413) in the case where I was running into this.