Fix “no signature key” in Ubuntu/Debian

Trying to update an Ubuntu 16.04 but was failing constantly because no signature key was there as you can see from the following warning: - The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 40976EAF437D05B5 NO_PUBKEY 3B4FE6ACC0B21F32 - The following signatures couldn't be verified because the public key is not…