fix icon resource fetching to work in Ubuntu 13.10

In the future, gobject introspection should be used for app indicator documentation as documented [here](http://developer.ubuntu.com/resources/technologies/application-indicators/), but for now this fixes the missing icon problem that appears in Ubuntu 13.10 as indicated in [this blog post](http://www.omgubuntu.co.uk/2013/12/hackertray-brings-hacker-news-menu-bar).

(I didn't test it in older Ubuntu versions, though it should still work)