PHPLIB-412: Use virtualized Xenial image in Travis CI

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

.travis.yml

 language: php
-dist: trusty
-sudo: false
+dist: xenial
 
 addons:
   apt:
@@ -23,6 +22,7 @@ matrix:
   fast_finish: true
   include:
     - php: 5.5
+      dist: trusty
     - php: 5.6
     - php: 7.0
     - php: 7.1