Add Object.assign() polyfill (#399)

config/polyfills.js

@@ -8,3 +8,7 @@ if (typeof Promise === 'undefined') {
 
 // fetch() polyfill for making API calls.
 require('whatwg-fetch');
+
+// Object.assign() is commonly used with React.
+// It will use the native implementation if it's present and isn't buggy.
+Object.assign = require('object-assign');

package.json

@@ -65,6 +65,7 @@
     "http-proxy-middleware": "0.17.0",
     "jest": "14.1.0",
     "json-loader": "0.5.4",
+    "object-assign": "4.1.0",
     "opn": "4.0.2",
     "path-exists": "3.0.0",
     "postcss-loader": "0.9.1",