Fix sprout init when.js error

Hey, I'm just opening this PR to have this documented before I delete the branch. There was an issue with when.js that caused an error when running `sprout init` from the CLI. It involved a regression in when's API with the `callbacks.call` function. The promise was being resolved even though inquirer's `prompt` function hadn't resolved the callback yet. Thankfully @briancavalier was able to help with a [patch](https://github.com/cujojs/when/pull/406) to when.js.