Upgrade Regenerator to v0.4.1

I revamped the way exceptions bubble up through `finally` blocks to follow the ES6 spec more closely, in particular by adopting the implementation strategy of associating a [completion record](https://people.mozilla.org/~jorendorff/es6-draft.html#sec-completion-record-specification-type) with every `try` statement.
Merge pull request #8 from benjamn/patch-1

Upgrade Regenerator to v0.4.1