Remove unnecessary call to generator when updating cron units

systemctl daemon-reload already takes care of deleting old unit files
and executing the generators again.
Use try-restart cron.target to only start it when it was active before.

Using try-restart cron.target gets only restarted when it was started before,
this way it won't get started when the administrator has decided to not enable
or start it.
Merge pull request #8 from schaal/remove-unnecessary-update-calls

Remove unnecessary call to generator when updating cron units