Make brew a relative path so boxen brew install works

I've been using your module for a while on a project of mine successfully (thanks, it works great!) until I started using [Boxen](https://boxen.github.com/) to setup my workstation. Boxen installs `brew` under its own path (`/opt/boxen/homebrew/bin/brew`).

According to the documentation I found on how Puppet loads commands in providers, it will search the path for a command if not qualified. I believe this solution should work for both Boxen-installed brew and the "regularly" installed brew.