Updating progress should yield consistent value.

[siteshen]

https://github.com/escaped/django-video-encoding/blob/master/video_encoding/backends/ffmpeg.py#L110-L112
yield value [0, 1]

            percent = round(time / total_time, 2)
            logger.debug('yield {}%'.format(percent))
            yield percent

while https://github.com/escaped/django-video-encoding/blob/master/video_encoding/backends/ffmpeg.py#L124
yield value 100

        yield 100