fix for gnome-shell 3.10

I've fixed the error caused by changes in ui.PanelMenu like in https://github.com/Tharre/gnome-shell-extension-sensors/commit/3127f3350c3b4d5f080421f71bc4ae07b9e7ee8a.
I don't know if it works on previous versions: I've got only 3.10 to test and I failed to find any documentation online.