Raise iOS deployment target. Qt 5 will only support iOS 5.0 and above.

Change-Id: I0feb7c2aadd8b107b3a89b1adabd48b7973bd11f
Reviewed-by: Tor Arne Vestbø <tor.arne.vestbo@digia.com>
Reviewed-by: Christian Kandeler <christian.kandeler@digia.com>