[Form][Serializer] Add missing conflicts for DI

Author: chalasr

composer.json

@@ -36,6 +36,7 @@
     },
     "conflict": {
         "phpunit/phpunit": "<4.8.35|<5.4.3,>=5.0",
+        "symfony/dependency-injection": "<3.2",
         "symfony/doctrine-bridge": "<2.7",
         "symfony/framework-bundle": "<2.7",
         "symfony/twig-bridge": "<2.7"