feat(promise-helpers): assert that argument is an array

[buschtoens]

Fixes #182.

FWIW ember-cli-sass / node-sass was giving me a major headache on my MacBook. I had to run as yarn --ignore-scripts to prevent the native node-sass build and then had to temporarily remove ember-cli-sass from the package.json in order to be able to start the test server.

I hope I didn't fuck up the yarn.lock. 😅

^{frankly, Sass needs to die in a fire... 🔥}

[buschtoens]

/cc @maxfierke

[maxfierke]

👍 thumbs up from me. yarn.lock looks alright to me. test failure is def unrelated.