The template for a CollectionViewController in Xcode comes with this line of code in viewDidLoad, which changes the specification in your storyboard.
// Register cell classes
self.collectionView!.register(UICollectionViewCell.self, forCellWithReuseIdentifier: reuseIdentifier)
Simply delete that line.
In older Swift versions the line is:
// Register cell classes
self.collectionView!.registerClass(UICollectionViewCell.self, forCellWithReuseIdentifier: reuseIdentifier)