docker-machine start

My first day using puppet, I noticed that in service.pp:22 you have:

```
$executable = "${boxen::config::homebrewdir}/bin/docker-machine"
```

In my machine (brand new Yosemite) the right line would be

```
$executable = "${boxen::config::homebrewdir}/opt/docker-machine/bin/docker-machine"
```

Not sure if this is a version problem, I resolved it with a symbolic link
