fixed ESLint error

Author: JamesMessinger

lib/util/meta.js

@@ -1,6 +1,6 @@
 'use strict';
 
-var META_SYMBOL = typeof Symbol === "function" ? Symbol('json-schema-ref-parser metadata') : "__metadata";
+var META_SYMBOL = typeof Symbol === 'function' ? Symbol('json-schema-ref-parser metadata') : '__metadata';
 
 /**
  * Safely sets meta property on the given object value