Configure new relic app name

NEW_RELIC_APP_NAME must be defined in order for newrelic.yml to work.

Since this seems to be a start-to-finish guide to getting discourse up, I think it makes sense to include this, even though it's in the existing [heroku new relic documentation](https://devcenter.heroku.com/articles/newrelic).

I was surprised to find that the heroku's environment doesn't already include it's app name.