CI: update Ubuntu from 18.04 to 19.04

Using 18.04 holds back the use of meson test suites in the CI infrastructure
(!216) and it's not likely to get an update to a more recent libinput version
anyway, so let's not bother with it here, even if it is an LTS.

Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>