Switch to my own images, with dependencies satisfied

Necessary PHP modules, as well as Composer, are provided in the new images: https://git.ethitter.com/docker/images/container_registry.

.gitlab-ci.yml

@@ -6,6 +6,7 @@ variables:
 cache:
   paths:
     - $HOME/.composer
+    - /root/.composer
 
 before_script:
   # Set up WordPress tests
@@ -30,8 +31,6 @@ PHPunit:PHP5.3:MySQL:
   script:
     - phpcs -n
     - phpunit
-  when: manual
-  allow_failure: true
 
 PHPunit:PHP5.6:MySQL:
   image: containers.ethitter.com:443/docker/images/php:5.6