MCPClient control: specify dep on ffmpeg 2.1.0 or later

We will be pushing out an update to the ffmpeg script post-release
which will use ffv1 level 3, and that requires ffmpeg 2.1.0 or later.
control: Depend on libavcodec-extra-55, not -53

-53 is an Ubuntu package that's associated with its older libav
release. It wouldn't actually be used by ffmpeg.