Change bowerDirectory assignment order in case .bowerrc doesn't contain `directory`

This fixes the rare case that someone has a `.bowerrc` file but it doesn't actually have a `directory` prop defined. Before, the code would continue with `this.bowerDirectory === undefined`
Then we just use that func and default:

```javascript
this.bowerDirectory = readJSONFile(path.join(this.project.root, '.bowerrc')).directory || 'bower_components';
```