Add intl polyfill for Safari 9

package.json

@@ -59,6 +59,7 @@
     "gh-pages": "github:rschamp/gh-pages#publish-branch-to-subfolder",
     "html-webpack-plugin": "^3.2.0",
     "immutable": "3.8.2",
+    "intl": "1.2.5",
     "jest": "^21.0.0",
     "keymirror": "0.1.1",
     "lodash.bindall": "4.4.0",

src/playground/index.jsx

+// Polyfills
 import 'es6-object-assign/auto';
 import 'core-js/fn/array/includes';
+import 'intl'; // For Safari 9
+
 import React from 'react';
 import ReactDOM from 'react-dom';