Specify all platform details in Sauce desired capabilities.

Not having all capabilities (platform, version, browserName) was causing
Sauce to return 'The environment you requested was unavailable.'

Weirdly, a Sauce session was still starting, but because the
browserPromise failed, the World lacked a reference to the browser.

This likely isn't a problem for local Selenium runs, but since the example includes Sauce explicitly I figured it was good to be working as is:P