Change tapped repo

`homebrew-` prefix is not needed. Current version complains even cask has already tapped.

```
boxen --noop
```
=>

```
Notice: /Stage[main]/Brewcask/Homebrew::Tap[caskroom/homebrew-cask]/Homebrew_tap[caskroom/homebrew-cask]/ensure: current_value absent, should be present (noop)
Notice: Homebrew::Tap[caskroom/homebrew-cask]: Would have triggered 'refresh' from 1 events
```