You have also to import the GPG signing key for the stable repository. Type in the terminal:
Code: Select all
gpg --keyserver keyserver.ubuntu.com --recv 976b5901365c5ca1
gpg --export --armor 976b5901365c5ca1 | sudo apt-key add -
Code: Select all
gpg --keyserver keyserver.ubuntu.com --recv 976b5901365c5ca1
gpg --export --armor 976b5901365c5ca1 | sudo apt-key add -