model.send(@progress, val) error if @progress is not specified

if we don't handle video converting progress it will perform model.send(nil) and raise error.

Original Error backtrace: 

Failed to transcode with FFmpeg. Check ffmpeg install and verify video is not corrupt or cut short. Original error: nil is not a symbol\n/home/aeliseev/pecaa/shared/bundle/ruby/1.9.1/gems/carrierwave-video-0.5.2/lib/carrierwave/video.rb:89:in `rescue in with_trancoding_callbacks