Support for "latest version" of a vendor/OS combo

[geekdave]

For evergreen browsers like Chrome and Firefox, it can be useful to pin a vendor/OS, but always test against the "latest" version to avoid frequent config change updates.

$ ./bin/guacamole --id=chrome_latest_Windows_2012_R2_Desktop          
[
  {
    "browserName": "chrome",
    "version": "44",
    "platform": "Windows 2012 R2"
  }
]

$ ./bin/guacamole --id=firefox_latest_Windows_2012_R2_Desktop
[
  {
    "browserName": "firefox",
    "version": "40",
    "platform": "Windows 2012 R2"
  }
]

Fixes https://github.com/TestArmada/guacamole/issues/6

[Maciek416]

Awesome