fix: ajv test to use id/$id correctly

Author: epoberezkin

.travis.yml

@@ -1,6 +1,6 @@
 language: python
 python: "2.7"
-node_js: "6"
+node_js: "9"
 install:
   - pip install tox
   - npm install

index.js

@@ -27,7 +27,8 @@ const SKIP = {
   if (draft == 7) {
     ajv = new Ajv({format: 'full'});
   } else {
-    ajv = new Ajv({format: 'full', meta: false});
+    const schemaId = draft == 4 ? 'id' : '$id';
+    ajv = new Ajv({format: 'full', meta: false, schemaId});
     ajv.addMetaSchema(require(`ajv/lib/refs/json-schema-draft-0${draft}.json`));
     ajv._opts.defaultMeta = `http://json-schema.org/draft-0${draft}/schema#`;
   }

package.json

@@ -21,7 +21,7 @@
   },
   "homepage": "https://github.com/json-schema-org/JSON-Schema-Test-Suite#readme",
   "devDependencies": {
-    "ajv": "^6.0.0-rc.0",
+    "ajv": "^6.0.0-rc.1",
     "json-schema-test": "^2.0.0",
     "mocha": "^3.2.0"
   }