travis: switch to 20.04
travis: switch to 20.04

Maybe a newer version of xephyr won't segfault randomly?

Signed-off-by: Tycho Andersen <6b523322678240882acbb4da5312a12d8cd0082d@tycho.ws>

ramnes/qtile,ramnes/qtile,qtile/qtile,qtile/qtile