Switch Travis CI to Ubuntu Xenial

lucaswerkmeister · lucaswerkmeister · commit 43e1f81cf94c · 2021-01-13T11:51:24.000+01:00
This seems to be needed for PHP 8.
diff --git a/.travis.yml b/.travis.yml
@@ -1,6 +1,6 @@
 language: php
 
-dist: trusty
+dist: xenial
 
 matrix:
   fast_finish: true
