The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 51716619E084DAB9
sudo su
gpg --keyserver keyserver.ubuntu.com --recv-keys 51716619E084DAB9
gpg -a --export 51716619E084DAB9 |apt-key add -
sudo su
gpg --keyserver keyserver.ubuntu.com --recv-keys 51716619E084DAB9
gpg -a --export 51716619E084DAB9 |apt-key add -