Added note about heroku killing idle apps

I thought there was a bug with shout until I dug around the heroku logs and found out that heroku kills idle apps after 1 hour of no http requests. Added a warning on the heroku deployment page in the documentation.

![screenshot](https://cloud.githubusercontent.com/assets/1098408/4692764/1fa607a0-5778-11e4-9044-d5493304b85e.png)