Fix libnotify >= 0.7.6

libnotify 0.7.6 changed the number of `Notification.add_action`
arguments (when called from python) from 6 to 5. The 6th argument should
never have been exposed and this commit hacks previous versions of
libnotify (`< 0.7.6`) to support 5 arguments.

Bug report: https://bugs.launchpad.net/ubuntu/+source/libnotify/+bug/1223401