Importing `es6-promise` had a side effect: this library replaces the
global Promise object (#304) (or tries to, #309). This is an intended
side effect from this library and while its version 4 should give us a
switch for this behavior, I don't know when it will be out (the master
branch of this project still auto replace the Promise).
I replaced it by `vow` which match the requirements:
- a Promise implementation
- a lightweight one
- works in IE6
- doesn't have too many dependencies
If a global Promise already exists, prefer it: a native promise is
likely to be better integrated anyway (unhandledRejection in node) and
some libraries (zone.js for example) replace global objects (#303).
I think it is enough for a semver minor version.