travis.yml: Update to Focal Fossa

It's better to use a newer version of the compiler which might catch
more issues in the code.

Signed-off-by: Thomas Huth <thuth@redhat.com>
Signed-off-by: Alexey Kardashevskiy <aik@ozlabs.ru>

.travis.yml

diff --git a/.travis.yml b/.travis.yml
index 5d0ac2d..375cabb 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -1,4 +1,4 @@
-dist: bionic
+dist: focal
 language: c
 
 addons: