Bumping ChefDK to version 0.3.1

```bash
$ brew cask audit chefdk --download
==> Downloading https://opscode-omnibus-packages.s3.amazonaws.com/mac_os_x/10.8/x86_64/chefdk-0.3.1-1.dmg
Already downloaded: /Library/Caches/Homebrew/chefdk-0.3.1-1.dmg
audit for chefdk: passed
```