Make rollbar variable optional

src/rollbar.js

@@ -1,7 +1,6 @@
-import Rollbar from 'rollbar';
-
 // Track error by rollbar.com
 if (location.host === 'preview.pro.ant.design') {
+  const Rollbar = require('rollbar').default;
   Rollbar.init({
     accessToken: '033ca6d7c0eb4cc1831cf470c2649971',
     captureUncaught: true,