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>
diff --git a/.travis.yml b/.travis.yml
index b497264..4b7947c 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -1,4 +1,4 @@
-dist: bionic
+dist: focal
 language: c
 cache: ccache
 git: