Bump Touch.Unit to fix compiler warning. (#1999)

commit spouliot/Touch.Unit@708348b0c1
Author: Rolf Bjarne Kvinge <rolf@xamarin.com>
Date:   Wed Apr 12 11:17:43 2017 +0200

    Fix compiler warning.

    Fix this compiler warning:

    > TouchRunner.cs(313,24): warning CS0618: `UIKit.UIAlertView.UIAlertView(string, string, UIKit.UIAlertViewDelegate, string, params string[])' is obsolete: `Use overload with a IUIAlertViewDelegate parameter'

external/Touch.Unit

@@ -1 +1 @@
-Subproject commit 16c5eddc9ed6906a21037c5a4501ba104faba948
+Subproject commit 708348b0c1a7c60c8e00e80df2a119b51ca2b85f