Support native Promise + ioredis v4

Created by: chaffeqa

Description

ioredis just released stable v4. Unless you use the Bluebird Promise switch, you run into an ambiguous error:

TypeError: scripts.updateDelaySet(...).then(...).return is not a function
    at node_modules/bull/lib/queue.js:374:18
    at runCallback (timers.js:693:18)
    at tryOnImmediate (timers.js:664:5)
    at processImmediate (timers.js:646:5)
    at process.topLevelDomainCallback (domain.js:121:23)
From previous event:
    at Queue._initProcess (node_modules/bull/lib/queue.js:362:8)

Minimal, Working Test code to reproduce the issue.

Any of the examples with "ioredis": "^4.0.0".

Bull version

3.4.6

Additional information