Remove extraneous return in PageStore dispatcher callback.

The dispatcher released by facebook (and included in this starter kit) doesn't use promises and doesn't use the return value of registered stores' dispatcher callbacks.

See https://github.com/facebook/flux/blob/master/src/Dispatcher.js#L220 and https://github.com/facebook/flux/blob/master/examples/flux-chat/js/stores/MessageStore.js#L108-134 for some Facebook evidence that this is no longer necessary (unfortunately they haven't been keeping the flux tutorial up-to-date).