Fix “following signatures couldn’t be verified because the public key is not available” in Ubuntu

If you get a message like this when updating with apt-get:

W: GPG error: http://extras.ubuntu.com precise Release: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 16126D3A3E5C1192

It can be fixed by just runing this with the key code in the error message:

$ sudo apt-key adv --keyserver keyserver.ubuntu.com --recv-keys 16126D3A3E5C1192

Ref: http://ubuntuforums.org/showthread.php?t=1869890