Fix PHP 5.3 test

WordPress 5.2 dropped support for PHP versions below 5.6, requiring that we test using WP 5.1.

.gitlab-ci.yml

@@ -23,6 +23,8 @@ before_script:
 
 PHP5.3:
   stage: test
+  variables:
+    WP_VERSION: '5.1'
   image: containers.ethitter.com:443/docker/images/php:5.3
   services:
     - mysql:5.6