Updated check for systemd

https://github.com/saltstack/salt/issues/4646

Based on http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=703600
and http://cgit.freedesktop.org/systemd/systemd/commit/?id=66e411811b8090

This will probably need testing on a variety of oses.

Thanks to Martin Pitt for the patch.