update to 0.9.4 with a little fix of info.chenyufei.cow.plist

Since the binary of `0.9.1` moved to a [new place], this tap won't work any more.

However after version bumping, I found an error in `info.chenyufei.cow.plist` that prevents launchd from launching `cow`, this fix works for me on `OS X 10.10`.

[new place]: http://dl.chenyufei.info/cow/0.9.1/cow-mac64-0.9.1.gz