Fix the webhook notifications tests
    
    Basically, when we changed year to 2019, the msg-id changed and
    that lead to different signatures hash.
    We're now asking the tests to stay on 2018, should make the tests
    consistent for the years to come!
    
    Signed-off-by: Pierre-Yves Chibon <pingou@pingoured.fr></pingou@pingoured.fr>