Use absolute URI in meta data schema $ref.

This fixes a test failure with jsonschema>=4.15.0. If "id" and "$ref" are
present at the same time, "id" is ignored in Draft4Validator. Because of this
a relative URI in "$ref" will not work, since there is no base URI to apply it
to.

Closes #58.

tests/test_jsonmerge.py

@@ -1970,8 +1970,7 @@ def test_reference_in_meta(self):
         schema = {'mergeStrategy': 'version'}
 
         meta_schema = {
-            'id': 'http://example.com/schema_1.json',
-            '$ref': 'schema_2.json#/definitions/meta'
+            '$ref': 'http://example.com/schema_2.json#/definitions/meta'
         }
 
         schema_2 = {