Deprecate 'return false' in event handlers

Fixes #2015. Wasn't sure how to handle `react-version` in _config.yml, given that this is a v0.12 milestone.
Though I suppose that capture the case where `listener.__reactBoundContext.constructor.displayName` is undefined...

```js
        var eventHandlerName =
          (listener.__reactBoundContext &&
           listener.__reactBoundContext.constructor.displayName) ||
          '<<anonymous>>';
```

is slightly better I suppose...