Fix version retrieval logic

[TheKevJames]

[kevin@aerobox(zsh):~] brew list --versions docker
docker 1.10.1 1.10.2 1.9.1
[kevin@aerobox(zsh):~] brew list --versions docker-machine
docker-machine 0.5.5 0.6.0

[jordigg]

I think this is solved already since they return the same output now for brew and brew cask

[TheKevJames]

To provide more context: my original worry in opening this issue was that the logic to get the currently installed version of a package may return any of the installed package versions rather than just the latest, since I originally wrote the logic without realizing that multiple versions could be installed. I don't think any changes we've made would have fixed this.