The relevant option is no longer available in the settings menu in Ubuntu 13.10; this has been reported as a bug (https://bugs.launchpad.net/ubuntu/+source/gnome-settings-daemon/+bug/1218322). However, it is still possible to swap these keys using the Gnome tweak tool.

First, install gnome-tweak-tool using sudo apt-get install gnome-tweak-tool from a console.

Then, run it using the command gnome-tweak-tool, or by typing "Tweak Tool" into the dash at the top-left.

From this tool, choose the "Typing" tab on the left. Then, from the dropdown next to "Ctrl key position" on the right, choose "Swap Ctrl and Caps Lock". This should swap those two keys.