I can tell you a cheat that will probably work if all else fails. Install the latest version of Mythbuntu, then install TVHeadEnd on top of that, then disable the Myth stuff from auto-starting at reboot. In my testing, TVHeadEnd will then find and use the HDHomeRun(s), and it's a heck of a lot less hassle to configure than the MythTV backend. I haven't installed this permanently yet (still waiting for them to fix the "bad skip issue" in the TVHeadEnd addon for Kodi, see http://github.com/kodi-pvr/pvr.hts/issues/92 ) so can't answer questions about it, but I did try it on an older machine long enough to confirm that it will work.
I'm assuming you've already tried the steps on the page at https://tvheadend.org/d/1587-complete-setup-guide-for-tvheadend-ubuntu-hdhomerun-tuner-xbmc-and-north-american-program-guides (particularly the part under "Install HDHomeRun Add-Ons"), which were apparently written for use with an older version of Ubuntu and have not been updated since? I know it says that won't work under Ubuntu 12.04 but maybe installing the HDHomeRun drivers would be enough to get it working in 14.04, I have no idea. If you try it, please let us know your results, because I'd be curious to know if that will work.