.sleep throws an error - uses undefined `Fiber` variable

[gustaff-weldon]

wdSync.sleep throws Fiber is undefined error

https://github.com/sebv/node-wd-sync/blob/master/lib/wd-sync.coffee#L88

I have fixed this by using fiber.constructor.yield()
Another option is exporting Fiber from make-sync and using it, but that's not the best way.

[sebv]

Fixed, thx for reporting.