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

...removed from the queue. Type safety would have been nice here ;-)
Expand the doc for this method. The actual sig is `def diff[B >: A](that: SeqLike[B])`. The new way of displaying the signatures in scaladoc is a big lie and I think it's more dangerous than useful.



> On 05.11.2014, at 18:33, Connor Doyle <notifications@github.com> wrote:
> 
> In src/main/scala/mesosphere/marathon/MarathonScheduler.scala:
> 
> > @@ -132,7 +132,7 @@ class MarathonScheduler @Inject() (
> >          }
> >  
> >          // put unscheduled tasks back in the queue
> > -        taskQueue.addAll(queuedTasks diff launchedTask.to[Seq])
> > +        taskQueue.addAll(queuedTasks diff launchedTask.map(_._1).to[Seq])
> Wow, I did not expect this given the signature of Seq.diff!
> 
> def diff(that: Seq[A]): Seq[A]
> Let's split this into two statements to force the compiler to verify the expected type:
> 
> val launchedTaskSeq: Seq[QueuedTask] = launchedTask.map(_._1).to[Seq]
> taskQueue.addAll(queuedTasks diff launchedTaskSeq)
> —
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>