fix promise typescript interface

Compile errors while I was using `Promise.all`.

`Thenable`'s .then was conflicting with `Promise`'s .then. This fixes how the then's fulfill is actually mandatory, and also that it doesn't have to return anything.

Also, why is the promise interface in *freedom-typescript-api*? Promises/A+ is a more general spec, and should probably go in DefinitelyTyped at some point..