MethodDescriptor: Fatal error: Call to a member function getParent()

If the `$node` is not an instance of `Descriptor\MethodDescriptor`, then `__invoke()` should return false.

While trying to build documentation, I am getting this error:

```
Load cache                                                         ..    0.024s
Preparing template "clean"                                         ..    0.008s
Preparing 16 transformations                                       ..    0.000s
Build "elements" index                                             ..    0.001s
Replace textual FQCNs with object aliases                          ..    0.009s
Resolve @link and @see tags in descriptions                        ..    0.001s
Build "packages" index                                             ..    0.002s
Collect all markers embedded in tags                               ..    0.000s
Build "namespaces" index and add namespaces to "elements"          ..    0.001s
Transform analyzed project into artifacts                          .. PHP Fatal error:  Call to a member function getParent() on a non-object in /srv/lib/phpDocumentor/phpDocumentor-2/vendor/phpdocumentor/phpdocumentor/src/phpDocumentor/Transformer/Router/UrlGenerator/Standard/MethodDescriptor.php on line 36
[2014-07-13 01:57:12] phpDocumentor.ALERT: Fatal Error (E_ERROR): Call to a member function getParent() on a non-object {"file":"/srv/lib/phpDocumentor/phpDocumentor-2/vendor/phpdocumentor/phpdocumentor/src/phpDocumentor/Transformer/Router/UrlGenerator/Standard/MethodDescriptor.php","line":36} []
<!DOCTYPE html>
<html lang="en">
<head>
```