Fix for issue #3186 - Allow Versioned::$versionableExtensions to be manipulated with Config::inst()->update()

This fix is allows Versioned::$versionableExtensions to be manipulated
by Config::inst()->update()
It would probably be better to demonstrate the config setting with yml rather than php, since that is the preferred mechanism. php assigned config doesn't benefit from caching, and it doesn't integrate as well with other modules which override the config of others modules.

You will also need to make sure the `$versionableExtensions` is private static, as is the requirement for config settings.