Error occurred when generating a gif image

[jiko797torayo]

This pull request helped me a lot.
https://github.com/streamio/streamio-ffmpeg/pull/162/files

Thank you for implementing it.

However, when generating a gif image,

movie = FFMPEG::Movie.new("hoge.mp4")
gif_image = movie.transcode(t.path, %W[-screenshot -seek_time 5 -r 5 -t 0.5 -vf scale=320:-1])

I get the following error.

Unrecognized option 'screenshot'.
Error splitting the argument list: Option not found