Accepting symfony/serializer#~4.0 (#319)

Since Laravel 5.6 works with symfony/serializer#~4.0 its time to upgrade?

Author: stephandesouza

composer.json

@@ -27,7 +27,7 @@
         "illuminate/support": "5.5.*|5.6.*",
         "illuminate/validation": "5.5.*|5.6.*",
         "illuminate/view": "5.5.*|5.6.*",
-        "symfony/serializer": "^2.7|^3.0"
+        "symfony/serializer": "^2.7|~3.0|~4.0"
     },
     "require-dev": {
         "phpunit/phpunit": "~5.0",