Updated chromedriver download methodology.

* Utilize S3 bucket XML instead of parsing HTML
* Encapsulate download URL logic in GoogleCodeParser
* Always re-fetch zip archive, because filenames all look the same
* Updated specs.

This fixes the issue I reported https://github.com/flavorjones/chromedriver-helper/issues/7

There is another Pull Request open https://github.com/flavorjones/chromedriver-helper/pull/8 but that simple patch will always returns chromedriver version 2.4 which will not alway be the newest.