new with —dry-run does not create new directory

fixes GH-1592
We generally wrap this `then` to another line (see [style guide](https://github.com/stefanpenner/ember-cli/blob/master/ARCHITECTURE.md#multi-line-return-statement)).  This would be something like:

```javascript
return exists(options.directoryName)
  .then(function() {
    if (exists){
      this.warnDirectoryAlreadyExists(options.directoryName);
    }
  }.bind(this));
```