Strongly recommend using 2.19.81, especially if I am going to help you out extending this symbol code library. The wording of unstable on the website is open source over modesty, a conventional trope. The latest releases are very stable, and I bang away at them all day on very very complex scores and even I cant crash them. There are lots of new features in the 2.19 series that I cannot imagine not wanting to have.
The Ubuntu repo is stuck at the nominated stable release. Just download from
lilypond.org.
Andrew