CI: run composer validate

So we are sure that the lock file is always in sync etc.

.gitlab-ci.yml

@@ -17,6 +17,7 @@ stages:
 .test_defaults: &test_defaults
   script:
     - sudo update-alternatives --set php "/usr/bin/${PHP}"
+    - composer validate
     - composer install
     - composer test
 

composer.json

 {
     "name": "dbp/relay-template-bundle",
+    "description": "A template bundle for the Relay API gateway",
     "type": "symfony-bundle",
     "license": "AGPL-3.0-or-later",
     "require": {