Installation fails if boot2docker not installed

```
** testing if installed package can be loaded
Error : .onLoad failed in loadNamespace() for 'kunteynir', details:
  call: boot2docker_shellinit()
  error: Running boot2docker locally requires boot2docker >= 1.3.0
```

Maybe this should be a separate check, triggered on calls to kunteynir functions, so we can at least install the package?
