I can't add the needed repository to install nvidia-graphics-drivers-319. I don't understand why the key is not trusted. See the output of the needed commands:

root@barry-III:/home/zombie# add-apt-repository ppa:ubuntu-x-swat/x-updates

 Updated versions of X.org drivers, libraries, etc. for Ubuntu.



This PPA is for stable upstream releases of X.org components.  If you're looking for something even more bleeding-edge, please see the xorg-edgers PPA.



While Ubuntu does not "officially/formally" support these packages, if you discover problems when installing these debs please feel free to report bugs to launchpad.  However, please make sure to clearly state that you are running packages from this PPA so we know the fixes need to come here.



If you are upgrading from one release to another with this PPA activated, please install the ppa-purge package and use it to downgrade everything in here beforehand. sudo ppa-purge ppa:ubuntu-x-swat/x-updates will do it.



[Directions for packaging drivers can be seen at https://wiki.ubuntu.com/X/DriverBuilding]

 More info: https://launchpad.net/~ubuntu-x-swat/+archive/x-updates

Press [ENTER] to continue or ctrl-c to cancel adding it



gpg: keyring `/tmp/tmph015_y/secring.gpg' created

gpg: keyring `/tmp/tmph015_y/pubring.gpg' created

gpg: requesting key AF1CDFA9 from hkp server keyserver.ubuntu.com

gpg: /tmp/tmph015_y/trustdb.gpg: trustdb created

gpg: key AF1CDFA9: public key "Launchpad PPA for Ubuntu-X" imported

gpg: no ultimately trusted keys found

gpg: Total number processed: 1

gpg:               imported: 1  (RSA: 1)

OK

root@barry-III:/home/zombie# apt-get install nvidia-graphics-drivers-319

Reading package lists... Done

Building dependency tree       

Reading state information... Done

E: Unable to locate package nvidia-graphics-drivers-319

root@barry-III:/home/zombie#