Super_L to activate the gnome main menu

For many releases of ubuntu I have used the tried and tested method of getting the windows key (super_L) to activate the gnome main menu. This does not work under ubuntu 15.04.

I run a script at log-in with the following commands:

xmodmap -e 'remove mod4 = Super_L'
xmodmap -e 'keycode 133 = F13'

Under system settings>keyboard>shortcuts>system I then change "show the applications menu" to F13 by hitting Super_L

This has worked for many previous releases but not 15.04. Under 15.04, xev shows me that the super_L is now F13 and F13 records under the "show the applications menu" line. A combination of keys such as cntrl F13, Alt F13, shift F13 can be selected to bring up the menu but not F13 on its own. Neither can I use any other Function key to bring up the menu unless it is in combination with another of those keys.

Is this a problem with my system or is this a change to ubuntu 15.04? And if a change, how do I now get Super_L to bring up the gnome main menu.