Lower QtQuick requirement further to 2.2

2.2 is required for the modifications we make to the Button objects.

resources/qml/OpenFilesIncludingProjectsDialog.qml

@@ -1,7 +1,7 @@
-// Copyright (c) 2015 Ultimaker B.V.
+// Copyright (c) 2017 Ultimaker B.V.
 // Cura is released under the terms of the AGPLv3 or higher.
 
-import QtQuick 2.6
+import QtQuick 2.5
 import QtQuick.Controls 1.1
 import QtQuick.Controls.Styles 1.1
 import QtQuick.Layouts 1.1