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`
Absolutely would. I wasn't covering malformed `.bowerrc` cases, just if it didn't have 'directory` defined.

e.g.

```json
{
  "analytics": false
}
```
I can certainly do so, but I think we shouldn't silently swallow any exception. c?