Fix handling of GTFS-realtime StopTimeUpdates with absolute times

GTFS-realtime StopTimeUpdate messages can include either a relative delay or an absolute time.  Both cases are already handled properly in `GtfsRealtimeTripLibrary.getTimeForStopTimeUpdate()`.  However, StopTimeUpdates with absolute times will never get that far, because `GtfsRealtimeTripLibrary.hasDelayValue()` only recognizes arrival and departure times with the `delay` field populated, not `time`.

I've tested this against the [GTFS-realtime feed](http://www.connexionz.co.nz/public_realtime_data_-_arlington) for [Arlington Transit](http://www.arlingtontransit.com/) (which supplies StopTimeUpdates with `time`) and found it to work well.