remove custom Promise handling

[gr2m]

All Node versions we currently support (Node 4, 6 and 8) have built-in Promise support, so we plan to remove the Promise options from the Constructor.

This like this will have no effect after v13.0.0

const Client = require('./../lib/index')
const github = new Client({
  Promise: Promise
})

Please let me know if this causes any trouble for you, maybe we can find a workaround that we can add to the migration guide :)

Related discussions are at #262, #258, #232

[gr2m]

This is done in #629 and will be closed once it's merged into master