Fix PHP 5.6 and 7.0 tests

Author: colinodell

.travis.yml

@@ -3,8 +3,6 @@ language: php
 dist: bionic
 
 php:
-  - "5.6"
-  - "7.0"
   - "7.1"
   - "7.2"
   - "7.3"
@@ -18,6 +16,10 @@ matrix:
       dist: trusty
     - php: 5.5
       dist: trusty
+    - php: 5.6
+      dist: xenial
+    - php: 7.0
+      dist: xenial
 
 install: travis_retry composer install --no-interaction --prefer-source