Don't be so specific, letting the API supply the point version.

.gitlab-ci.yml

@@ -24,7 +24,7 @@ before_script:
 PHPunit:PHP5.3:MySQL:
   stage: test
   variables:
-    WP_VERSION: 5.1.1
+    WP_VERSION: '5.1'
   image: containers.ethitter.com:443/docker/images/php:5.3
   services:
     - mysql:5.6