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.
@crm416 ugh sorry, I derped. This is better as `listener.__reactBoundContext.constructor.displayName || '<<anonymous>>'`. It makes more sense to go find the `displayName` this way, than to look for it on the handler, since the handler can come from a mixin (in which case the method's `displayName` is kinda useless no matter how we do it).

So if you change this line to that, it won't depend on the other PR anymore, and I can pull this in.