TypeError: setTimeout(...).unref is not a function

Hi!
I have some problems with using stoppable on node v8.7.0

here are couple of stack traces:

```
TypeError: setTimeout(...).unref is not a function

at Immediate.setImmediate [as _onImmediate] (node_modules/stoppable/lib/stoppable.js:33:39)
at runCallback (timers.js:785:20)
at tryOnImmediate (timers.js:747:5)
at processImmediate [as _immediateCallback] (timers.js:718:5)
```
```
TypeError: setTimeout(...).unref is not a function

at Immediate.setImmediate [as _onImmediate] (node_modules/stoppable/lib/stoppable.js:33:39)
at runCallback (timers.js:785:20)
at tryOnImmediate (timers.js:747:5)
at processImmediate (timers.js:718:5)
at _combinedTickCallback (internal/process/next_tick.js:131:7)
at process._tickCallback (internal/process/next_tick.js:180:9)
```
I use babel, jest, flow and lots of other things, so it's hard to say is it problem of stoppable or not.