Quote PHP versions & add Trusty dist

PHP 5.4 and 5.5 were failing build process on the default Xenial distribution. Attempting to use Trusty.

Author: AkenRoberts

.travis.yml

@@ -1,17 +1,17 @@
+dist: trusty
 language: php
 
 sudo: false
 
 matrix:
     include:
-        - php: 5.4
+        - php: '5.4'
           env: phpunit_exclude_groups=datetimeinterface
-        - php: 5.5
-        - php: 5.6
-        - php: 7.0
-        - php: 7.1
-        - php: 7.1
-        - php: 7.2
+        - php: '5.5'
+        - php: '5.6'
+        - php: '7.0'
+        - php: '7.1'
+        - php: '7.2'
 
 cache:
     directories: