If you’ve just migrated to XCode 7 and your app is appearing slightly letter-boxed with black bars at the top and bottom then this might help.
Apparently the root issue is that storyboards now expect to have a launch image and something in the process of launching your app goes awry if there isn’t one which results in the storyboard not sizing properly.
There’s a more technical explaination out there, but that’s the gist of it.
To sort it, all you need to do is add a launch image (Files, Add, iOS, User interface, Launch image). AND, in my case, set the project to use it: