Mark pickled existential test as passing.

The problem with pickled existential flag has been fixed upstream
in Scala 2.10 (the one sbt is using right now) so let's mark this
test as passing.

Note, that the fix went into Scala 2.9.3 so it's passing with that
version of Scala as well.

Closes #616. We don't need to work-around this on sbt side because
sbt already upgraded to Scala 2.10 where this is fixed.

Closes #623. Since sbt switched to Scala 2.10 we don't need a
specific version of Scala for this test.