Nrefused, send no warning if systemd <239

[juju4]

When installing systemd_exporter on Ubuntu 18.04, there is a spam of logs related to https://github.com/povilasv/systemd_exporter/blob/master/systemd/systemd.go#L644 installation done with https://github.com/cloudalchemy/ansible-systemd-exporter

I think that systemd version should be tested once and if below, don't issue warning as expected. Or have a config option to silence message.

Thanks