fix youtube-dl shasum

existing shasum appears to be incorrect:

```
==> Upgrading 1 outdated package, with result:
youtube-dl 2014.08.29
==> Upgrading youtube-dl
==> Downloading https://downloads.sf.net/project/machomebrew/Bottles/youtube-dl-2014.08.29.mavericks.bottle.tar.gz
######################################################################## 100.0%
Error: SHA1 mismatch
Expected: a157d91e208fc4d5d7b18d2679d636ce7e00bec3
Actual: 8e83f99f38fedfbac2800c6b68bc1bf8ecc28dcb
Archive: /Library/Caches/Homebrew/youtube-dl-2014.08.29.mavericks.bottle.tar.gz
To retry an incomplete download, remove the file above.
Warning: Bottle installation failed: building from source.
==> Downloading https://yt-dl.org/downloads/2014.08.29/youtube-dl-2014.08.29.tar.gz
######################################################################## 100.0%
==> make PREFIX=/usr/local/Cellar/youtube-dl/2014.08.29
==> Caveats
To use post-processing options, `brew install ffmpeg`.
Bash completion has been installed to:
  /usr/local/etc/bash_completion.d
==> Summary
🍺  /usr/local/Cellar/youtube-dl/2014.08.29: 7 files, 688K, built in 4 seconds
```