How to Install flatpak on Linux Mint 17.1

GIMP 2.10 came out and I'm really excited to use it. The website suggests installing it with flatpak. According to flatpak's website, Linux Mint 18 and forward have it already installed, but as I'm using Linux Mint 17.1, I do not have it. I tried following the Ubuntu installation requirements for flatpak, as well as some other forums, which suggest entering the following into the terminal: apt add-repository ppa:alexlarsson/flatpak apt update apt install flatpak After entering the last command, the terminal prints out: Reading package lists... Done Building dependency tree Reading state information... Done E: Unable to locate package flatpak Any ideas on how I can install flatpak or GIMP 2.10 without using flatpak?