allow symfony 4

Author: taylorotwell

composer.json

@@ -15,7 +15,7 @@
         "illuminate/contracts": "~5.1",
         "illuminate/support": "~5.1",
         "psy/psysh": "0.7.*|0.8.*",
-        "symfony/var-dumper": "~3.0"
+        "symfony/var-dumper": "~3.0|~4.0"
     },
     "require-dev": {
         "phpunit/phpunit": "~4.0|~5.0"