Account for mismatch between /browsers and /workers in BrowserStack API v3

* fixWorker: Check for worker.browser.os instead of worker.browser.version because
  - Mobile doesn't have browser version
  - This property was renamed in v3 to browser_version
* fixWorker: Update property names to v3 versions
* fixWorker: Fill null values for worker properties missing in /workers from /browsers
* handleBrowser: Exclude worker.browser property from index due to upstream bug

Fixes #41.