Ubuntu 18.04 (Bionic) ships Neovim 0.2.2, so that explains why you'll get that version by default.
You can use a PPA from the “Neovim PPA” team to install the latest Neovim stable on your Ubuntu 18.04 (also available for 16.04 and 19.10)
Use these commands to configure your system to always include this PPA in updates:
sudo add-apt-repository ppa:neovim-ppa/stable
sudo apt-get update
It's odd that you have a Neovim v0.5.0-nightly installed in your system. If you're getting it from a deb package or from a different PPA, you might want to make sure you uninstall that before you'll be able to install the one from the Stable PPA.