Treat RHEL 7 and later like Fedora w/r/t iptables

RHEL 7 replaces legacy init with systemd, and must be treated like
Fedora w/r/t iptables persistence.  This patches checks
operatingsystemrelease in addition to operatingsystem in order
to differentiate between RHEL 7 and earlier releases.

Both RHEL 7 and recent Fedora releases require the iptables-services
package to support the commands in lib/puppet/util/firewall.rb.

This also corrects the path to /usr/libexec/iptables/iptables.init.
correct spelling of "&&" to "and"
fix errors noted by travis-ci

I like explicitly quoted strings, but apparently Travis doesn't.
Merge pull request #338 from larsks/master

Treat RHEL 7 and later like Fedora w/r/t iptables