Lib contains class syntax, not compatible with IE11

https://unpkg.com/exceljs@3.2.0/dist/exceljs.js

line 17612:

![image](https://user-images.githubusercontent.com/6134547/67266184-24e0a880-f4e2-11e9-9c86-abb94abf72e5.png)
