PHPC-1333: Use virtualized Xenial image in Travis CI

Use a Trusty image for PHP 5.5, since it is not available on Xenial.

Author: jmikola

.travis.yml

@@ -1,5 +1,5 @@
 language: php
-sudo: false
+dist: xenial
 
 addons:
   apt:
@@ -21,6 +21,7 @@ matrix:
         - TESTS=tests/atlas.phpt
         - TEST_PHP_ARGS="-q -s output.txt -x --show-diff"
     - php: 5.5
+      dist: trusty
     - php: 5.6
     - php: 7.0
     - php: 7.1