Looks like the builders are still running Ubuntu 12.04. 12.04 is now five years old, and technically is end-of-lifed in a couple of weeks (April 7 2017). I'm suspecting we ought to move off it - not in the middle of a release cycle, of course. We also list that 12.04 is a "supported" release for development on the wiki.
https://wiki.ubuntu.com/Releases
