Updating to jsdom-13

[p4t5h3]

Analog to #24 the jsdom version 13 released a month ago should be supported, too.

[Jonas Amundsen]

Thanks for letting me know! Fixed with release of v6.1.5.

(I've also come to know that one can subscribe to new releases on Github, which I've done for jsdom. I hope I'll be better at doing this in a timely manner by myself.)