* add support custom CIContextFactory.use it fix crash in iOS8

[stat0s2p]

read http://stackoverflow.com/questions/39570644 answer
create CIContext instance in object-c and set it to QRCode.CIContextFactory static field in iOS8.
example:

if #available(iOS 9.0, *){
  return
}
QRCode.CIContextFactory = {
  return CIContext.cso_context(options: nil)
}