If you attach a `ref` to an element using a string (`ref='foo'`) and use `ReactTestRenderer` you get the `"Stateless function components cannot be given refs"` warning

See https://github.com/facebook/react/issues/7371#issuecomment-237816352

This is because `getPublicInstance` returns `null` and `attachRef` requires that the instance be non-null in the warning invariant.

I think we can either return a simple instance instead of `null` or use some additional check in `attachRef` to see if we're dealing with `ReactTestRenderer`. The first option seems reasonable, given the comment in the current code: 

``` js
ReactTestComponent.prototype.getPublicInstance = function() {
  // I can't say this makes a ton of sense but it seems better than throwing.
  // Maybe we'll revise later if someone has a good use case.
  return null;
};
```

cc @spicyj 

this._updateBatchNumber = null;
    }
  },
  /**
   * Perform an update to a mounted component. The componentWillReceiveProps and
   * shouldComponentUpdate methods are called, then (assuming the update isn't
   * skipped) the remaining update lifecycle methods are called and the DOM
   * representation is updated.
   *
   * By default, this implements React's rendering and reconciliation algorithm.
   * Sophisticated clients may wish to override this.
   *
   * @param {ReactReconcileTransaction} transaction
   * @param {ReactElement} prevParentElement
   * @param {ReactElement} nextParentElement
   * @internal
   * @overridable
   */
  updateComponent: function(
    transaction,
    prevParentElement,
    nextParentElement,
    prevUnmaskedContext,
    nextUnmaskedContext
  ) {
    var inst = this._instance;
    invariant(
      inst != null,
      'Attempted to update component `%s` that has already been unmounted ' +
      '(or failed to mount).',