On iOS 9, `viewWillTransitionToSize:withTransitionCoordinator:` requires super. Also renames the `rotating` property to `transitioning` to better match the new API context.
On iOS 9, `viewWillTransitionToSize:withTransitionCoordinator:` requires super. Also renames the `rotating` property to `transitioning` to better match the new API context.

slackhq/SlackTextViewController,kobim/SlackTextViewController,kobim/SlackTextViewController,slackhq/SlackTextViewController