Fix PHP 5.4 and 5.5 on CI

.travis.yml

@@ -9,7 +9,9 @@ matrix:
     - php: 5.3
       dist: precise
     - php: 5.4
+      dist: trusty
     - php: 5.5
+      dist: trusty
     - php: 5.6
     - php: 7.0
     - php: 7.1