Fix dependency issue

I get the following error on a pristine Boxen installation:

>```
Error: /Stage[main]/Elasticsearch::Package/Exec[/opt/boxen/homebrew/bin/plugin --install elasticsearch/elasticsearch-lang-javascript/2.1.0]: Could not evaluate: Could not find command '/opt/boxen/homebrew/bin/plugin'
```

This change addresses that.
Merge pull request #1 from envato/homebrew-dependency

Fix dependency issue