Fix ReferenceError: destDir is not defined

I've tried to use the plugin today and stumbled upon the fact it throws a ReferenceError:

```
[broccoli-debug] [AssetRewrite tree] ReferenceError: destDir is not defined
ReferenceError: destDir is not defined
  at .../node_modules/broccoli-source-map/index.js:12:33
  at Array.forEach (native)
  at SourceMapInliner.updateCache (.../node_modules/broccoli-source-map/index.js:31:19)
...
```

The PR fixes this. Hope this helps.