Merge pull request OAI#410 from APIs-guru/ref_fix

Fix '$ref' validation.

Author: webron

schemas/v2.0/schema.json

@@ -1492,6 +1492,7 @@
     },
     "jsonReference": {
       "type": "object",
+      "required": ["$ref"],
       "additionalProperties": false,
       "properties": {
         "$ref": {