This fixed my issue with the following error.

Code:
W: GPG error: http://ppa.launchpad.net intrepid Release: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY CB2F6C86F77B1CA9
W: You may want to run apt-get update to correct these problems
Code:
gpg --keyserver keyserver.ubuntu.com --recv CB2F6C86F77B1CA9
gpg --export --armor CB2F6C86F77B1CA9 | apt-key add -