If it were me, I'd just add Mattia Rizzolo's ppa for Sigil and be done with it. He's the one who packages Sigil for Ubuntu and Debian. He'd be the one who prepared the 0.9.5 version you're using now. He's got a Sigil 0.9.8 package for 16.04.
Code:
sudo add-apt-repository ppa:mapreri/sigil
sudo apt-get update
Then to install ...
Code:
sudo apt-get install sigil
If any of that makes you uncomfortable, I'll walk you through removing your current Sigil installation, downloading/installing my stand-alone 0.9.8 package, and migrating your current preferences/plugins.