Update when.js dependency to version 3.2.3

I think it's worth updating to the latest version (3.2.3) of the when.js promise library.  I know there's been some discussion (#968) of switching to Bluebird but in the interim it would be nice to take advantage of the work that's gone into newer versions of `when`.

Lots of improvements in 3.0+ in terms of performance, memory usage, and logging (unhandled rejections).
(https://github.com/cujojs/when/releases)

Considerable improvements in relation to Bluebird according to Bluebird's own benchmarks from where it was at a few months ago: https://github.com/petkaantonov/bluebird/blob/master/benchmark/stats/latest.md

I've gone through the upgrading from 2.x to 3.x guide (https://github.com/cujojs/when/blob/master/docs/api.md#upgrading-to-30-from-2x) and made the necessary changes (very few), and I've been using it on my development repo for a few weeks now with no issue (and it really is better at catching unhandled rejections).