update systemd

systemd/tunasync-snapshot-gc.service

@@ -4,7 +4,7 @@ Requires = network.target
 After = network.target
 
 [Service]
-Type=simple
+Type=oneshot
 ExecStart=/home/tuna/.virtualenvs/tunasync/bin/python -u /home/tuna/tunasync/tunasync_snapshot_gc.py -c /etc/tunasync.ini
 
 [Install]