I get the following error when running the above command line:
GPG error: http://ppa.launchpad.net trusty Release: The following signature could not be verified because there is no public key: NO_PUBKEY 6D1D8367A3421AFB
sudo apt-key adv --keyserver keyserver.ubuntu.com --recv-keys 6D1D8367A3421AFB
Just replace 6D1D8367A3421AFB with the currently present keys.