Remove use of flags from Procfile

This env for [$PORT](https://github.com/heroku/busl/blob/46b36295ff35b4f7a8f1436de2f0aeadf8e39c2b/util/config.go#L10) and [$REDIS_URL](https://github.com/heroku/busl/blob/46b36295ff35b4f7a8f1436de2f0aeadf8e39c2b/broker/redis.go#L21) is already being read from inside the code, so no need to pass these in from the Procfile too. 

This caused confusion because `REDIS_URL` and `REDISTOGO_URL` are being read in in different places. I only saw the `REDIS_URL` one, and when I dropped `REDISTOGO_URL`, shit broke. This makes it much clearer what is going on and removes the vendor-specific `REDISTOGO_URL` dependency.