Warning in React Fiber

Need rewrite TreeNode to class.

```
Warning: Stateless function components cannot be given refs. Attempts to access this ref will fail.

Check the render method of `DropTarget(TreeNode)`.
    in TreeNode (created by DropTarget(TreeNode))
    in DropTarget(TreeNode) (created by ReactSortableTree)
    in div (created by ReactSortableTree)
    in ReactSortableTree (at PageTree.js:57)
```