Fix misbehaving promise polyfill

[benjamn]

The phantomjs-prebuilt package (typically installed here) now depends on a Promise polyfill package called es6-promise, which misbehaves by overwriting the global.Promise constructor. This becomes a problem when Meteor tools code attempts to call Promise.prototype.await that are added by our (well-behaved) runtime setup code, because global.Promise is no longer the same as the version of Promise that we augmented.