Switch to xenial to have more stable dependencies

See: https://travis-ci.community/t/travisci-chrome-version-has-regressed-back-to-v62/8603/2?u=banzaiman

Author: benoittgt

.travis.yml

@@ -40,9 +40,6 @@ before_script:
 
 script: "script/run_build 2>&1"
 
-# In order to install old Rubies, we need to use old Ubuntu distibution.
-dist: trusty
-
 matrix:
   include:
     # Rails 6.0 builds >= 2.5.0