unable to resolve symbolic references

hi,

im trying to get the commit at my current HEAD:

```
return repo.getReference('HEAD').then(function(ref) {
  return repo.getCommit(ref.target());
});
```

but im getting an error:

```
TypeError: Object object has no method 'resolve'
    at node_modules/nodegit/lib/repository.js:68:24
```

nodegit version: current master (e9fa6f1fa7eb)
