Comment 9 for bug 1421184

Tim Edwards, there's another way to work-around this that might be easier. I should've added a comment on it.

For the command-line client ONLY: http://packages.ubuntu.com/xenial/unison
For both graphical and command-line client: http://packages.ubuntu.com/xenial/unison-gtk
Make sure to download the right architecture from below, most likely 'amd64' for 64-bit, or 'i386' for 32-bit. You shouldn't need any of the dependencies; Ubuntu 14.04 has new-enough versions.

If you have both packages installed (no need), you'll have to upgrade both or remove the old 'unison' package as 'unison-gtk' seems to offer the same program.

I've had success directly downloading the .deb packages for Unison 2.48.3-1 and installing them on my desktop and laptop running Ubuntu 14.04 with latest updates. Nothing seems to break, just a slight sensible re-arrangement of the GUI buttons. This isn't the best idea, but it's a viable work-around for now. A proper backport to trusty-updates would be better.