[BUGFIX beta] Update to latest Backburner.js.

From https://github.com/ebryn/backburner.js/pull/54:

```
   Fix a bug where debounced item in index 0 isn't
   removed unless debounce for that target/method is called again, but
   then it's in index 0 and then isn't removed
```

[Compare view](https://github.com/ebryn/backburner.js/compare/d6bc31f978eb0ef0c8288c1bef9d08420caff00a...afa5f6e22541772fbc702e2e1d7a3afc480b66b1)