A few small tweaks that should reduce the number of Heroku API calls and help apps not hit the Heroku API rate limit.

I've changed the logic for when to scale up or down slightly to avoid fetching the worker numbers from Heroku API when it's not necessary. 

The current behaviour has not been changed at all. The only consequence is less API calls which helps with Heroku API rate limits and should also slightly improve performance.

This should help with Issue #33.