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 

var mouse = renderer.getInstance();
    mouse.handleMoose();
    expect(renderer.toJSON()).toEqual({
      type: 'div',
      props: {},
      children: ['moose'],
    });
  });
  it('updates types', function() {
    var renderer = ReactTestRenderer.create(<div>mouse</div>);
    expect(renderer.toJSON()).toEqual({
      type: 'div',
      props: {},
      children: ['mouse'],
    });
    renderer.update(<span>mice</span>);
    expect(renderer.toJSON()).toEqual({
      type: 'span',
      props: {},
      children: ['mice'],
    });
  });
  it('updates children', function() {
    var renderer = ReactTestRenderer.create(
      <div>
        <span key="a">A</span>
        <span key="b">B</span>
        <span key="c">C</span>
      </div>
    );