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 

if (__DEV__) {
    warning(
      element === null || element === false || React.isValidElement(element),
      '%s(...): A valid React element (or null) must be returned. You may have ' +
      'returned undefined, an array or some other invalid object.',
      Component.displayName || Component.name || 'Component'
    );
    warning(
      !Component.childContextTypes,
      '%s(...): childContextTypes cannot be defined on a functional component.',
      Component.displayName || Component.name || 'Component'
    );
  }
}
function shouldConstruct(Component) {
  return !!(Component.prototype && Component.prototype.isReactComponent);
}
function isPureComponent(Component) {
  return !!(Component.prototype && Component.prototype.isPureReactComponent);
}
function measureLifeCyclePerf(fn, debugID, timerType) {
  if (debugID === 0) {