Use proper import instead of require.

Author: oschwede

src/worker.js

@@ -1,7 +1,8 @@
 import jsPDF from 'jspdf';
 import html2canvas from 'html2canvas';
 import { objType, createElement, cloneNode, toPx } from './utils.js';
-var Promise = require('es6-promise').Promise;
+import es6promise from 'es6-promise';
+var Promise = es6promise.Promise;
 
 /* ----- CONSTRUCTOR ----- */