Detect Node 6 in test environment and disable ES2015 transforms

Created by: gaearon

As mentioned by @cpojer this can significantly speed up the tests. We only need to leave the CommonJS transform from ES6 because Node doesn't support modules yet.

https://twitter.com/cpojer/status/783651892320571394

If you intend to work on this please write in a comment here. If somebody else is already working on it please don't try to beat them. If you later change your mind it's fine but please comment here so somebody else can take the issue.