Prefer time field in a GTFS-realtime StopTimeEvent.

The [GTFS-realtime spec](https://developers.google.com/transit/gtfs-realtime/reference#StopTimeEvent) dictates that the `time` field should be preferred over `delay` if both are present, but OneBusAway's handling of the fields was inverted from the spec.

Discovered in onebusaway/onebusaway-application-modules#78.