fix path with windows back slash with path.join

this fixed and issue in `Windows`:
When using `path.join`, path separators get transformed to `\`, they are then interpreted as escaping characters and raise errors.

```javascript
define('path\to\module');
```
it shoud be either
```javascript
define('path\\to\\module');
```
or
```javascript
define('path/to/module');
```
This fix revert back unix separators after join.