Swift 4.0 updated version, without "bangs" and with proper variable naming.

UIImage+Rotation.swift

extension UIImage {

    func rotate(degrees: CGFloat) -> UIImage? {
        let degreesToRadians: (CGFloat) -> CGFloat = {
            return $0 / 180.0 * .pi
        }

        let rotatedViewBox = UIView(frame: CGRect(origin: CGPoint(x:0, y:0), size: size))
        let transformation = CGAffineTransform(rotationAngle: degreesToRadians(degrees))

        rotatedViewBox.transform = transformation
        let rotatedSize = rotatedViewBox.frame.size

        UIGraphicsBeginImageContext(rotatedSize)
        guard let temporaryBitmap = UIGraphicsGetCurrentContext(), let transformingImage = cgImage else {
            return nil
        }

        temporaryBitmap.translateBy(x: rotatedSize.width / 2.0, y: rotatedSize.height / 2.0)
        temporaryBitmap.rotate(by: degreesToRadians(degrees))
        temporaryBitmap.scaleBy(x: 1.0, y: -1.0)

        let temporaryRect = CGRect(x: -size.width / 2, y: -size.height / 2, width: size.width, height: size.height)

        temporaryBitmap.draw(transformingImage, in: temporaryRect)

        guard let rotatedImage = UIGraphicsGetImageFromCurrentImageContext() else {
            return nil
        }

        UIGraphicsEndImageContext()

        return rotatedImage
    }
}