Fixing resque-mailer compatability and not scaling down when jobs are running

The original branch of the autoscaler is not compatible with resque-mailer, this pr includes a fix for that. There is a pull request on the ajmurmann master for that: https://github.com/ajmurmann/resque-heroku-autoscaler/pull/8

Also it has a check so it only scales down when it is time to scale down to zero, never from say 5 to 3 since that risks killing jobs that take a long time to run.

Two tests are failing on this branch but I did not really understand how to fix them.

This is the branch running on sc.com so it could be good to have these changes in master.