Change-chromecast-dns-to-opendns-on-openwrt

ssh中运行： 这样就可以解决chromecast无法升级的毛病了.
 * iptables -t nat -A PREROUTING -d 8.8.8.8 -j DNAT --to-destination 208.67.222.222
 * iptables -t nat -A PREROUTING -d 8.8.4.4 -j DNAT --to-destination 208.67.220.220