Link to the ES6 spec

Quote from https://github.com/domenic/promises-unwrapping#status :

> This proposal has progressed to the Draft ECMAScript 6 Specification, which is available for review on the official ECMAScript wiki. When referencing the promises specification, you should reference the draft ECMAScript 6 spec, and not this repository.
Merge pull request #52 from DavidBruant/patch-1

Link to the ES6 spec