enable libx264 and libfdk-aac

[peteruhrig]

For proper playing in Chrome and generally for compatibility with our other datasets, we need to export h.264 video. Since we use libfdk-aac for our other audio streams, we may want to use that here, too. The current ffmpeg included in the Case installation is compiled without libx264 and libfdk-aac.
However, this is just a temporary solution. We need to modify the entire dataflow and avoid re-encoding where possible. See separate issue.