The official repository has changed. Based on the new wiki instructions, here is how I installed inxi on my Xubuntu 12.04 LTS machine:

sudo add-apt-repository ppa:unit193/inxi
sudo apt-get update
sudo apt-get install inxi

inxi is available without a ppa.

However, you must enable the universe repository.

After you enable the universe repository, run the following commands to install inxi:

sudo apt update
sudo apt install inxi

Additionally, updates via the inxi -U command are disabled by default. If you have problems with the current package managed version, you can update to a more current version of the script by running the following commands:

sudo sed -i 's/ATE=false/ATE=true/g' /etc/inxi.conf
sudo inxi -U

To view the current version, run the following command:

inxi -V | grep inxi

Click here for more information.