fix selenium.jar downloads

Some users end up with a corrupt selenium.jar when using this module. I can't reproduce the issue. This is an attempt to fix it.

- switch to the [download](https://www.npmjs.org/package/download) module for actual downloading and zip extraction; primary
- switch to google storage to find the latest selenium.jar
- refactor so that selenium and chromedriver are complete separate parallel paths
- validate downloads with checksums