Add the version 4 of symfony in composer.json for the yaml versio

Author: Julien Grelet

composer.json

@@ -98,7 +98,7 @@
         "symfony/security-bundle":      "~2.3|~3.0",
         "symfony/options-resolver":     "~2.3|~3.0",
         "symfony/form":                 "~2.3|~3.0",
-        "symfony/yaml":                 "~2.3|~3.0",
+        "symfony/yaml":                 "~2.3|~3.0|~4.0",
         "kriswallsmith/buzz":           "~0.13"
     },