[BUGFIX] Use trees in broccoli

The Brocfile allows you to pass in alternate names and locations for directories with the `trees` property. However, I was having issues with this when trying to put the `app, config, vendor, etc` directories in their own directory, like so:
```
trees: {
  app: 'ember/app',
  tests: 'ember/tests',
  vendor: 'ember/vendor',
  styles: 'ember/app/styles',
}
```

This should fix that issues by replacing hardcoded instances like `'vendor'` and `'tests'` with `this.trees.vendor` and `this.trees.tests`.