add set_heroku_vars_changed and run_heroku_config_if_settings_changed to...

... roboconf.sh

This works on marketing release and prep-release. It will reconfigure Heroku settings (and restart Heroku to do so) only if the Heroku settings have changed. I would like to extend this to other scripts.

The basic idea is to replace:
  echo_cmd heroku config:set $HEROKU_CONFIG_ADD_CONSTANTS --app "$app"
with:
  curl -so .roboconf.sh https://raw.github.com/hedgeyedev/roboconf/master/roboconf.sh
  source .roboconf.sh
  run_heroku_config_if_settings_changed