Fix deprecated '--join' option in the CoffeeScript 1.8.0

[see](https://github.com/jashkenas/coffeescript/commit/1cd48a9f5d38e823434c6c00ba41846270535a47)
something like that maybe?

```python
coffees = ' '.join([
    os.path.join(DIR_STATIC, script)
    for script in scripts if script.endswith('.coffee')
  ])
if is_windows:
  coffees = coffees.replace(' ', ',')
```