Try this
gpg --keyserver keyserver.ubuntu.com --recv-keys F2A8428D3C354953
gpg --export --armor F2A8428D3C354953 | sudo apt-key add -
CAUTION: In the last command above, do not miss the minus ( - ) sign at the end after word “add”
NEXT:
sudo apt-get update
in case you want to upgrade rbfeeder give following command
otherwise skip the following command
sudo apt-get install rbfeeder `