Comment 15 for bug 1263540

I know the last comment is a month old now, but google is still driving people here for an answer. I found this on the web:
http://www.namhuy.net/3116/how-to-fix-gpg-error-no_pubkey-in-ubuntu.html

$ sudo apt-key adv --keyserver keyserver.ubuntu.com --recv-keys 40976EAF437D05B5

Run this command for each hex key you need to download. replace "40976EAF437D05B5" with the appropriate key.

I had to do it three times, but it worked for me.