Run `apt-get update` for Ubuntu to fix errors seen because of an
out-of-date index:
E: Failed to fetch [...] 404 Not Found [IP: 40.119.46.219 80]
E: Failed to fetch [...] 404 Not Found [IP: 40.119.46.219 80]
E: Failed to fetch [...] 404 Not Found [IP: 40.119.46.219 80]
Signed-off-by: Lukas Fleischer <lfleischer@calcurse.org>
b0d94d4e84