Refactor task

Change task name from `wprev` to `version`

Roots work perfectly when `querystring` enable ~~but useless when `querystring` disable~~.

```php
wp_enqueue_style( $handle, $src, $deps, $ver, $media );
wp_register_script( $handle, $src, $deps, $ver, $in_footer );
```

When `querystring` not present, version task will use filename revving for default and ensure `wp_enqueue_style` (`$media`) and `wp_register_script` set to `null`.

Fixed roots/grunt-wp-version#1