Support "latest-1" (and maybe -2?) browser version

[geekdave]

We already have support for the latest version of a browser.

Some folks have been asking for a long time for n-1 or n-x version support and perhaps we can provide it while waiting for SauceLabs to implement it.

[Maciek416]

Love this idea. This is a cool way to have a sweeping window of tiers without having to maintain any specific versioning at all.

[geekdave]

SauceLabs now supports this natively: http://sauceio.com/index.php/2016/03/new-browser-version-shortcuts

We can also unwind #7 and just delegate to the native supported version.