[BUGFIX] Only use vertxNext() when it is not undefined.
[BUGFIX] Only use vertxNext() when it is not undefined.

In a Titanium app environment, browserWindow is undefined but a require function is availabl. When used with Webpack to bundle the library we take the wrong path, and setTimeout is the only option.


from https://github.com/stefanpenner/es6-promise/pull/241
tildeio/rsvp.js,tildeio/rsvp.js