add node 13, and 14 to travis, and fix stream close event listener called multiple times on node 14

[AbdelrahmanHafez]

fixes #8835

[AbdelrahmanHafez]

For some reason stream.on('close', cb); calls the callback twice on node 14, not sure if this is a bug in mongoose or node 14, will dig some more into this.

The test passes by replacing stream.on(...) with stream.once(...), but I am sure there are users out there who are relying on stream.on(...) working correctly.

[vkarpov15]

Works for me. I haven't been able to find much about the changes to streams in Node 14 beyond this article: https://medium.com/@nodejs/node-js-version-14-available-now-8170d384567e . But since tests pass I think this should work.