chore: update dependencies

Author: jleeothon

composer.json

@@ -26,8 +26,8 @@
   ],
   "require": {
     "php": "^7.0",
-    "illuminate/contracts": "^5.4",
-    "symfony/yaml": "^3.4",
+    "illuminate/contracts": "^5.6",
+    "symfony/yaml": "^4.1",
     "justinrainbow/json-schema": "^5.2"
   },
   "require-dev": {