caffeine-systray: Follow my own bugfix fork.

caffeine-systray linked to a specific upstream commit, before systray support
was dropped. I've forked the proyect myself, branching off that commit.

This package now follows that fork, for simplicity.
caffeine-systray: Use a stable zipball.

This package was previously using the latest git HEAD. It's better to use the
latest stable release, and have a separate package for the latest git release.