Updated list of available platforms

OS X 10.7 is not supported, Windows 8.1 was added by Sauce Labs
Assign browsers configuration object only if it is specified.

Having no browsers configuration in Dalekfile.json will never fallback to the default (desiredCapabilities) as an error will occur while trying to access an non existing object.
Restructured platform verification

Currently the first element in the platform array will never be evaluated.
Removed a bit of complexity from '_verfiyBrowserConfig'
Merge pull request #3 from bauschan/master

fix(config): Updated list of available platforms