Documentation: Update QtQuick import value to 2.4

There are new APIs in Qt 5.4, the import version needed is 2.4.

Change-Id: I1c52c6a50362f539670d9ad00e03e57208234a17
Reviewed-by: J-P Nurmi <jpnurmi@theqtcompany.com>
Reviewed-by: Topi Reiniö <topi.reinio@digia.com>