Utilise `composer update` plutôt que `composer install` en CI.

.gitlab-ci.yml

@@ -13,7 +13,7 @@ stages:
     - php -v
     - composer -V
   script:
-    - composer install $COMPOSER_OPTS
+    - composer update $COMPOSER_OPTS
     - php -l src/*.php
     - vendor/bin/php-cs-fixer fix --verbose --dry-run --using-cache=no
     - vendor/bin/phpunit