There are some repositories that seem to be mantaining Sigil at this moment for Ubuntu 15.04:
i2p: https://launchpad.net/~i2p.packages/+archive/ubuntu/i2p
sudo add-apt-repository ppa:i2p.packages/i2p
sudo apt-get update
sudo apt-get install sigil
xiota: https://launchpad.net/~xiota/+archive/ubuntu/ppa
sudo add-apt-repository ppa:xiota/ppa
sudo apt-get update
sudo apt-get install sigil
mapreri: https://launchpad.net/~mapreri/+archive/ubuntu/sigil
sudo add-apt-repository ppa:mapreri/sigil
sudo apt-get update
sudo apt-get install sigil
Though it is important to note that i2p.packages repository doesn’t contain only Sigil, and updating after adding it may upgrade other packages.