Fixes 'latest' keyword not always returning highest available version

The list of available versions is not always ordered, meaning `latest` just taking the last element of the list doesn't result in the truly latest browser in all cases.

You can see this behavior in master currently with the following `.zuul.yml` browsers field:

```
browsers:
    - name: chrome
      platform: Windows 2012 R2
      version: latest
```

The result should be a test run in Chrome 35 on Windows 2012 R2, but instead a test is run in Chrome 31.

I added a guarantee that the `beta` version won't get selected when a user specifies `latest`, since that seemed like more intuitive behavior to me and because with the sorting strategy used `beta` would now always be the value of `latest` for Chrome.