Build with Node 6 fails

[franzliedke]

I currently can't build the flarum/core JS with Node 6+.

Because of nodejs/node#5348, Node.JS v6+ throw exceptions when a path is not a string.

Apparently, we somehow incorrectly initialize the gulp-babel plugin, because that's the error I get:

TypeError in plugin 'gulp-babel'
Message:
    Path must be a string. Received undefined

Building with Node 5 works for now. :)