Use trusty for PHP 5.6 to fix issue with memcache extension

Author: W0rma

.travis.yml

@@ -1,7 +1,6 @@
 language: php
 
 php:
-    - 5.6
     - 7.0
     - 7.1
     - 7.2
@@ -24,6 +23,9 @@ matrix:
         - php: 5.5
           dist: trusty
           sudo: required
+        - php: 5.6
+          dist: trusty
+          sudo: required
 
 # faster builds on new travis setup not using sudo
 sudo: false