travis.yml: Update from Bionic to Focal

The newer version of QEMU in Focal should help to run more tests
with TCG (which will be enabled in later patches).

Message-Id: <20200924161612.144549-2-thuth@redhat.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>

.travis.yml

-dist: bionic
+dist: focal
 language: c
 cache: ccache
 git: