Added es6-symbol polyfill

Author: auser

package.json

@@ -51,5 +51,8 @@
     "react-test-renderer": "^15.3.0",
     "should": "^11.1.0",
     "debug": "^2.2.0"
+  },
+  "dependencies": {
+    "es6-symbol": "^3.1.0"
   }
 }