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`
This will still call `.directory` against the return value of `fs.readFileSync` wouldn't it?

I think we need:

```javascript
// this goes outside the constructor
function readJSONFile(path) {
  var result;

  try {
    var contents = fs.readFileSync(path, { encoding: 'utf8' });

    result = JSON.parse(contents);
  } catch(e) {
    result = {};
  }

  return result;
};
```