Use Facebook Platform Versioned URL's

By Configuration the Plugin user can choose which version of the Facebook API to use, defaulting to the unversioned API.

>Useful for testing the behaviour of the application on new versions of the Facebook Platform that will eventually become the default when using the unversioned URL (ie. for testing the impact of changes on the permissions definitions).

According to [Facebook's Platform Versioning documentation](https://developers.facebook.com/docs/apps/versions)