Airtime-liquidsoap should start before Apache

When launched, Liquidsoap tries to connect to Airtime API, served by Apache. Because rc2.d script pache2 is started (S91) after airtime-liquidsoap (S20), it fails at each boots of Ubuntu. Thanks to Monit (S99), though, airtime-liquidsoap is soon relaunched. But it should start right from the first try. ;-)

```bash
# Current
## Start
/etc/rc2.d/S20airtime-liquidsoap
/etc/rc2.d/S20airtime-media-monitor
/etc/rc2.d/S20airtime-playout
/etc/rc2.d/S91apache2
/etc/rc2.d/S99monit
## Stop
/etc/rc0.d/K01monit
/etc/rc0.d/K09apache2
/etc/rc0.d/K20airtime-liquidsoap
/etc/rc0.d/K20airtime-media-monitor
/etc/rc0.d/K20airtime-playout

# Suggested
## Start
/etc/rc2.d/S20airtime-media-monitor
/etc/rc2.d/S20airtime-playout
/etc/rc2.d/S91apache2
/etc/rc2.d/S92airtime-liquidsoap
/etc/rc2.d/S99monit
## Stop
/etc/rc0.d/K01monit
/etc/rc0.d/K08airtime-liquidsoap
/etc/rc0.d/K09apache2
/etc/rc0.d/K20airtime-media-monitor
/etc/rc0.d/K20airtime-playout
```

Note: Maybe there is the same kind of problem for Media-Monitor and Playout, but I didn't noticed anything yet.