update debian installation
Issue generated from Tuleap's migration script. Originally submitted by: sergio (sergio)
"Instead of using this command a keyring should be placed directly in the /etc/apt/trusted.gpg.d/ directory with a descriptive name and either "gpg" or "asc" as file extension."
Update the https://ring.cx/en/download/gnu-linux plaese, which says to use apt-key.