Latitude 3550: Cannot activate middle button emulation on Ubuntu 14.04

I am using dconf-editor and/or

> gsettings set org.gnome.settings-daemon.peripherals.mouse middle-button-enabled true

as well as created /usr/share/X11/xorg.conf.d/middle-mouse-button.conf with the following contents:

 Section "InputClass"
     Identifier "middle button emulation class"
     MatchIsPointer "on"
     Option "Emulate3Buttons" "on"
 EndSection

but I cannot use middle button from the touchpad at all. External mouse middle button works fine

> uname -a
Linux kna5 3.13.0-73-generic #116-Ubuntu SMP Fri Dec 4 15:31:30 UTC 2015 x86_64 x86_64 x86_64 GNU/Linux