brew: Use salt.utils.version_cmp to determine latest package version installed.

Although only one version is ever linked, the output from `brew list --versions` has output similar to the following.
```
autoconf 2.69
ffmpeg 2.1.3 2.1.4
percona-server 5.6.15-rel63.0
freetype 2.5.2 2.5.3
jenkins 1.553 1.554
```