Update GPG keys Ubuntu repositories

Sometimes we may need to update the public keys of some Ubuntu repositories. A symptom is the following error:

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 4FEC45DD06899068`

The solution is very simple.

sudo apt-key adv --recv-keys \
    --keyserver keyserver.ubuntu.com \
     06899068