Fix num_desired_heroku_workers not returning integers.

With fast resque jobs and multiple workers, we've seen an edge case
where `#num_desired_heroku_workers` returns the WORKER_SCALE array instead
of a number.  Somehow the `Scale.job_count` is zero despite having just
enqueued a job, so our theory is that another worker has already grabbed
the job from the queue while the API call was happening.