Fix apt-get update GPG error

I added a new source to my /etc/apt/sources.list file and got this message from apt-get update:

W: GPG error: http://raspberrypi.collabora.com wheezy Release: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY ED4BF9140C50B1C5

Clearly, I needed to import that GPG key into apt. Here's how that is done:

sudo gpg --keyserver hkp://subkeys.pgp.net --recv-keys ED4BF9140C50B1C5
sudo gpg --export --armor ED4BF9140C50B1C5 | sudo apt-key add - 

YMMV, so replace the hex id of the key with the one that your system is complaining about.