Fix small subtle bug in GROUP_BY constraint.

@guenter @krishnap

Commit message should be sufficiently descriptive.
I'll take your word for it. Seems like your Scala-fu is better than mine.
On Aug 2, 2014 9:38 AM, "Dario Rexin" <notifications@github.com> wrote:

> In src/main/scala/mesosphere/mesos/Constraints.scala:
>
> > @@ -51,7 +51,7 @@ object Constraints {
> >
> >        // Order groupings by smallest first
> >        val orderedByCount = groupedTasks.toSeq.sortBy(_._2.size)
> > -      def minValue = orderedByCount.headOption.map(_._1).getOrElse("")
> > +      def minValue = orderedByCount.headOption.map(_._1.get).getOrElse("")
>
> How would throwing a NoSuchElementException help in this case? It would
> probably be a good to add logging if it's None, but Option.get is an
> unsafe method that shouldn't even exist.
>
> —
> Reply to this email directly or view it on GitHub
> <https://github.com/mesosphere/marathon/pull/421/files#r15729932>.
>