It seems things are packaged somewhat differently on Debian/Ubuntu than on Fedora.  I think on Ubuntu, the required package is: gir1.2-gtk3.0 https://launchpad.net/ubuntu/precise/+package/gir1.2-gtk-3.0

Since we don't really know what distro the user will be using, I'm inclined to change this to:

```
"The Gtk3 backend requires that the GObject introspection bindings for Gtk 3 are installed."
```

That should give a user enough to Google something by.