add missing real_ip_header CF-Connecting-IP;

Mentioned further up the page at http://www.babaei.net/blog/2013/03/09/getting-real-ip-addresses-using-nginx-and-cloudflare/ and required as per https://support.cloudflare.com/hc/en-us/articles/200170706-Does-CloudFlare-have-an-IP-module-for-Nginx- but forgotten from the script.