How do I install Sigil in Ubuntu 15.04?

Ubuntu 16.04 (and later)

Sigil is in official repositories for Ubuntu 16.04, so you can install it without adding any 3rd party PPA:

sudo apt-get install sigil

The latest version Sigil 0.8.2 is available only here at the moment:

sudo add-apt-repository ppa:i2p.packages/i2p
sudo apt-get update
sudo apt-get install sigil