2.0: stricter validation of additionalProperties

Following clarification of 'additionalProperties' for a schema by
@webron:
swagger-api/swagger-codegen#1318 (comment)

We now accept only `false` or a schema for `additionalProperties`.
`false` is the new default for `additionalProperties` (instead of `{}`
which means "anything is accepted" in JSON Schema).

schemas/v2.0/schema.json

@@ -991,10 +991,12 @@
               "$ref": "#/definitions/schema"
             },
             {
-              "type": "boolean"
+              "description": "false",
+              "type": "boolean",
+              "enum": [ false ]
             }
           ],
-          "default": {}
+          "default": false
         },
         "type": {
           "$ref": "http://json-schema.org/draft-04/schema#/properties/type"
@@ -1588,4 +1590,4 @@
       }
     }
   }
-}
+}