callback-baseline parallel benchmark is broken in Node 6.x

[edsadr]

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Please answer the questions the best you can:

1. What version of bluebird is the issue happening on?
   3.4.1

2. What platform and version? (For example Node.js 0.12 or Google Chrome 32)
   Node v6.x

3. Did this issue happen with earlier version of bluebird?
   yes

The callback-baseline test in parallel benchmark is broken, it fails with this callstack:

benchmarking ./madeup-parallel/callbacks-baseline.js
timers.js:314
    throw new TypeError('"callback" argument must be a function');
    ^

TypeError: "callback" argument must be a function
    at exports.setTimeout (timers.js:314:11)
    at Object.dummy_n (/Users/edsadr/Sites/bluebird/benchmark/lib/dummy.js:7:13)
    at Timeout.onComplete (/Users/edsadr/Sites/bluebird/benchmark/madeup-parallel/callbacks-baseline.js:21:24)
    at tryOnTimeout (timers.js:224:11)
    at Timer.listOnTimeout (timers.js:198:5)
benchmarking ./madeup-parallel/callbacks-caolan-async-parallel.js
timers.js:314
    throw new TypeError('"callback" argument must be a function');