Update http kernel constraint to address security vulnerability.

Author: bangpound

composer.json

@@ -14,7 +14,7 @@
         "symfony/config": "~2.8|~3.0|~4.0|~5.0",
         "symfony/dependency-injection": "~2.8|~3.0|~4.0|~5.0",
         "symfony/expression-language": "~2.8|~3.0|~4.0|~5.0",
-        "symfony/http-kernel": "~2.8|~3.0|~4.0|~5.0",
+        "symfony/http-kernel": ">=4.4.50|~5.0",
         "symfony/yaml": "~2.8|~3.0|~4.0|~5.0"
     },
     "replace": {