iptables: support iptables older than 1.4.11

this checks the version of iptables, if it's older it'll use an iptables list
command to check if a rule exists, otherwise it'll use the newer -C command

Fixes #91
Merge pull request #92 from mindscratch/issue-91-support-old-iptables

iptables: support iptables older than 1.4.11