Replace deprecated ActorRef.isTerminated

[drozzy]

How can we replace the deprecated isTerminated method? It is used in FlyingBehaviour.scala on line 93:

def prepComplete(data: Data): Boolean = {
    data match {
      case FlightData(c, _, _, _, s) =>
        if (!c.isTerminated &&
          s.heading != -1f && s.altitude != -1f)
          true
        else false
      case _ =>
        false
    } 
  }

[danluu]

Sorry, I basically haven't used scala since doing this. I'd be happy to take a PR, but I haven't been keeping up with things enough to know what's deprecated and what's new.