supervisor doesn't know that you have added a program. This is answered on serverfault, do the following:
supervisorctl reread
supervisorctl update
By the way, it's easier to maintain configuration files using the conf.d syntax. In other words, create a file called /etc/supervisor/conf.d/artisan.conf. Everything else is the same, but it's easier to version control your configuration files and sync them to machines on setup.
