Update nginx.conf after installing nginx package

This is another solution proposed for #23. Quoting #23,
> The default of daemon off; is causing salt-minion to hang when executing the nginx service state (i.e. when calling /etc/init.d/nginx start) on Ubuntu 14.04 and 12.04.

However, changing the default value of `daemon` doesn't solve the root cause; the minion would still hang if someone explicitly used `daemon: True` for whatever reason.