Apt-get update fails because the PGP keys for the repository have expired.
$ sudo apt-get update W: An error occurred during the signature verification. The repository is not updated and the previous index files will be used. GPG error: http://repo.mysql.com jessie InRelease: The following signatures were invalid: KEYEXPIRED 1487236823 KEYEXPIRED 1487236823 KEYEXPIRED 1487236823 W: Failed to fetch http://repo.mysql.com/apt/debian/dists/jessie/InRelease W: Some index files failed to download. They have been ignored, or old ones used instead.
Update the necessary key by adding it to your keystore
$ sudo apt-key adv --keyserver pgp.mit.edu --recv-keys A4A9406876FCBD3C456770C88C718D3B5072E1F5