Using selenium 2.43 and latest webdriver-http-sync

So far it looks like 2.43 is the golden version - latest Firefox works *and* phantom. But it requires an update to `webdriver-http-sync` so that `/value` is processed according to the latest JSONWire spec. Game plan:

* Release a major version of `selenium-download`
* Update this PR with `selenium-download@^2.0.0`
* Merge & release this as a minor testium bump
* Update the testium2 branch with `selenium-download@^2.0.0`