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.
Merge pull request #1 from G5/concurrency-bug

Fix num_desired_heroku_workers not returning integers.