On Heroku, SERVER_NAME should equal $DYNO

[nateberkopec]

https://devcenter.heroku.com/articles/dynos#local-environment-variables

Maybe if $DYNO is present, we can just hook that up automatically. I wonder what a sane way to detect if we're on Heroku would be? Look for env variables with HEROKU in them? Look for a .heroku folder? Or is the presence of DYNO fine?

[nateberkopec]

Although probably a "private API" in a sense, we could check for the presence of /etc/heroku, which we already do to get the release.