Switch to preserved ffmpeg 1.0.0~rc1 tarball
The tarball got recreated by GitHub, thus invalidating the previous
checksums. This PR replaces the URL by the preserved original tarball
that came from the OPAM cache.
Error message:
#=== ERROR while fetching sources for ffmpeg-av.1.0.0~rc1, ffmpeg-avcodec.1.0.0~rc1 and ffmpeg-avutil.1.0.0~rc1
OpamSolution.Fetch_fail("https://github.com/savonet/ocaml-ffmpeg/archive/v1.0.0-rc1.tar.gz (Bad checksum, expected md5=996d69434972553ecf3ae04e95d5c09d)")