minor #31428 Fix tests on Travis for 4.2 (fabpot)

This PR was merged into the 4.2 branch.

Discussion
----------

Fix tests on Travis for 4.2

| Q             | A
| ------------- | ---
| Branch?       | 4.2
| Bug fix?      | no
| New feature?  | no <!-- don't forget to update src/**/CHANGELOG.md files -->
| BC breaks?    | no     <!-- see https://symfony.com/bc -->
| Deprecations? | no <!-- don't forget to update UPGRADE-*.md and src/**/CHANGELOG.md files -->
| Tests pass?   | yes    <!-- please add some, will be required by reviewers -->
| Fixed tickets | n/a
| License       | MIT
| Doc PR        | n/a

<!--
Write a short README entry for your feature/bugfix here (replace this comment block.)
This will help people understand your PR and can be used as a start of the Doc PR.
Additionally:
 - Bug fixes must be submitted against the lowest branch where they apply
   (lowest branches are regularly merged to upper ones so they get the fixes too).
 - Features and deprecations must be submitted against the master branch.
-->

Commits
-------

1b128b0a7b fixed tests on Travis

.travis.yml

@@ -1,6 +1,6 @@
 language: php
 
-dist: trusty
+dist: xenial
 
 git:
     depth: 2
@@ -13,6 +13,7 @@ addons:
         - slapd
         - zookeeperd
         - libzookeeper-mt-dev
+        - rabbitmq-server
 
 env:
     global: