Fix run method

The previous run method would cause the installation to fail when running `script/boxen` to provision the box without loading Boxen's environment (`source /opt/boxen/env.sh`).

```shell
$ sudo -E -u user /opt/boxen/homebrew/bin/brew list | grep cask
brew-cask
$ sudo -E -u user /opt/boxen/homebrew/bin/brew tap | grep cask
caskroom/cask
$ sudo -E -u user /opt/boxen/homebrew/bin/brew cask install alfred
Error: Unknown command: cask
```

The new run method has been copied from [boxen/homebrew method](https://github.com/boxen/puppet-homebrew/blob/master/lib/puppet/provider/package/homebrew.rb).

This is an addition to #15. Now, puppet-brewcask works out of the box.