Update atk submodule ref

The previous ref was resulting in:

```bash
Cloning into 'atk'...
remote: Counting objects: 2206, done.
remote: Compressing objects: 100% (1760/1760), done.
remote: Total 2206 (delta 381), reused 2206 (delta 381)
Receiving objects: 100% (2206/2206), 3.43 MiB | 1.67 MiB/s, done.
Resolving deltas: 100% (381/381), done.
fatal: reference is not a tree: 5ff8245450d459f451112a405aa34d7b6ee46e04
Unable to checkout '5ff8245450d459f451112a405aa34d7b6ee46e04' in submodule path 'atk'
```

I resolved it following the "Inside, Out" part of this recipe: [Git submodule head 'reference is not a tree' error](http://stackoverflow.com/questions/2155887/git-submodule-head-reference-is-not-a-tree-error)