switch to request-sync to support windows

[request-sync](https://github.com/ForbesLindesay/request-sync) is a wrapper for [http-sync](https://github.com/dhruvbird/http-sync) and [http-sync-win](https://github.com/ForbesLindesay/http-sync-win) that provides windows support.

This should allow webdriver-http-sync to work better on windows!