fixed bug in the MarathonScheduler that led to launched tasks not being ...

...removed from the queue. Type safety would have been nice here ;-)
Wow, I did not expect this given the signature of `Seq.diff`!

```scala
def diff(that: Seq[A]): Seq[A]
```

Let's split this into two statements to force the compiler to verify the expected type:

```scala
val launchedTaskSeq: Seq[QueuedTask] = launchedTask.map(_._1).to[Seq]
taskQueue.addAll(queuedTasks diff launchedTaskSeq)
```