Fixed web UI redirection by proxy.

Instead of redirecting web requests to the other master, we'll just
proxy everything.
Bumped scala lib versions.
Fixed 'app doesn't exist' bug.

When a task starts and finishes before marathon has a chance to handle
the status updates, the app gets lost.  We should make sure we know
about the app as soon as a task is launched.
Fixed another task accounting bug.

When apps are shut down or scaled down, we need to wait until we get
acknowledgement before removing those tasks from Marathon's state.