Bump minimal requirements

Author: jderusse

composer.json

@@ -26,7 +26,7 @@
         "symfony/expression-language": "~3.4|~4.0",
         "symfony/http-foundation": "~3.4|~4.0",
         "symfony/ldap": "~3.4|~4.0",
-        "symfony/validator": "~3.4|~4.0",
+        "symfony/validator": "^3.4.31|^4.3.4",
         "psr/log": "~1.0"
     },
     "conflict": {