MDL-68392 travis: Fix validation warnings and infos

Can verify that this file passes the validation @
https://config.travis-ci.com/explore

.travis.yml

@@ -2,15 +2,15 @@
 # process (which uses our internal CI system) this file is here for the benefit
 # of community developers git clones - see MDL-51458.
 
-sudo: required
-
 # We currently disable Travis notifications entirely until https://github.com/travis-ci/travis-ci/issues/4976
 # is fixed.
 notifications:
   email: false
 
 language: php
 
+os: linux
+
 dist: xenial
 
 services:
@@ -41,7 +41,7 @@ env:
     # Perform an upgrade test too.
     - DB=pgsql    TASK=UPGRADE
 
-matrix:
+jobs:
     # Enable fast finish.
     # This will fail the build if a single job fails (except those in allow_failures).
     # It will not stop the jobs from running.