Fix use of boost::shared_ptr in DartLoader::parseSkeleton()

`urdf::parseURDF()` create and return `boost::shared_ptr<urdf::ModelInterface>`, which is reference counting pointer, and the `boost::shared_ptr::get()` returns a pointer of `urdf::ModelInterface to skeletonModel`. Then, the returned `boost::shared_ptr<urdf::ModelInterface>` is destroyed, and `skeletonModel` is also destroyed since no other `boost::shared_ptr<urdf::ModelInterface>` references it.

@tobiaskunz Could you review this pull request?