Fix: returning promises created in handlers

[D1plo1d]

See: http://bluebirdjs.com/docs/warning-explanations.html#warning-a-promise-was-created-in-a-handler-but-was-not-returned-from-it

This pull request resolved the warnings I received when using iterall with Bluebird installed as the global Promise object (global.Promise = require('bluebird')):

(node:16490) Warning: a promise was created in a handler at [REDACTED]/node_modules/iterall/index.js:650:23 but was not returned from it, see http://goo.gl/rRqMUw
    at Function.Promise.cast ([REDACTED]/node_modules/bluebird/js/release/promise.js:196:13)
    at [REDACTED]/node_modules/iterall/index.js:650:23
    at runCallback (timers.js:773:18)
    at tryOnImmediate (timers.js:734:5)
    at processImmediate [as _immediateCallback] (timers.js:711:5)

[leebyron]

Unfortunately adding a return here creates a memory leak. A line like this used to exist and was removed to fix the leak. Bluebird also recommends explicitly returning null from the handler, so lets try that instead.

[D1plo1d]

Thanks!