Fix bug in line detection of iptables-restore --test.

When having errors in iptables rules, I get the following error:
```
[2014-07-14T21:46:59+02:00] ERROR: ruby_block[test-iptables] (simple_iptables::default line 65) had an error: RuntimeError: iptables-restore exited with code 1 while testing new rules
STDOUT:

STDERR:
iptables-restore: line 31 failed
```

It doesn't show what rule failed, and the file is deleted afterwards. I am running on Debian 7.

This commit solves my problem.

Best regards,
Kim