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()
Since versioned itself is an extension, could you not apply class-specific versionableExtensions to a single class such as:

```php
Config::inst()->update('SiteTree', 'versionableExtensions', array(/*.. stuff specific to SiteTree */));
```

There's nothing wrong with using Versioned as a config source in its own right, but it would allow for greater flexibility to take advantage of the available config-inheritance function of extensions.

It'd require only the below change:

```php
$versionableExtensions = $this->config()->versionableExtensions;
```