Fix get default route if (close GH-1)

If a VPN is mounted another default route exist (with a different metric)

❯ sudo ./firewall.sh
Bad argument `link'
Try `iptables -h' or 'iptables --help' for more information.
❯ ip -4 r|awk '/^default/ {print $5}'
wlan0
link
❯ ip -4 r |head -2
default via 192.168.1.1 dev wlan0
default dev eth0  scope link  metric 1002
Merge pull request #2 from enovance/bug/1/sbadia

Fix get default route if (close GH-1)