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

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