Add validation steps for PHP 5.4 and 5.3

.gitlab-ci.yml

@@ -30,6 +30,18 @@ validate:5.5:
   script:
     - bash ci/php_lint.sh ./
 
+validate:5.4:
+  image: php:5.4
+  stage: validate
+  script:
+    - bash ci/php_lint.sh ./
+
+validate:5.3:
+  image: php:5.3
+  stage: validate
+  script:
+    - bash ci/php_lint.sh ./
+
 test:7.1:
   image: php:7.1
   stage: test