Remove non-portable sort usage from update script

It failed with the following error on OSX:

```
+ do_releasetag ffmpeg n
+ cd ffmpeg
++ git tag
++ grep '^n[0-9]'
++ sort -V
++ tail -n 1
sort: invalid option -- V
Try `sort --help' for more information.
+ version=
+ git checkout --detach refs/tags/
fatal: git checkout: --detach does not take a path argument 'refs/tags/'
```

I couldn't find any better sort solution but the special case for ffmpeg. :(