There are a number of ways to do this.
1. According to: https://wiki.ubuntu.com/XorgCtrlAltBackspace/, Ctrl+Alt+Backspace was replaced with AltGR-Sysreq-K (I believe AltGr would be the Right Alt key).
2. The command "setxkbmap -option terminate:ctrl_alt_bksp" should re-enable it. You can add this command to your startup apps or to ~/.xprofile (if the file doesn't exist, create it. Also make sure its executable). But this is not a system-wide change - only for the user that has this set.
3. As root, create the file /usr/share/X11/xorg.conf.d/2-dontzap.conf with the following content:
(note: you'll need to restart X for this to take effect.)
Option "DontZap" "false"