Formatter not working

I'm using Sublime Text 3.1 and I cannot get my formatter to work.

I've enabled `format_on_save` in my package settings but whenever I save i get the following message in the ST console:

```
no such context .nest_curly_and_self in Packages/Elixir/Syntaxes/Elixir.tmLanguage
Traceback (most recent call last):
File "/Applications/Sublime Text.app/Contents/MacOS/sublime_plugin.py", line 1072, in run_
return self.run(edit)
File "/Users/johan/Library/Application Support/Sublime Text 3/Installed Packages/Elixir.sublime-package/mix_format_file.py", line 49, in run
File "./python3.3/subprocess.py", line 819, in __init__
File "./python3.3/subprocess.py", line 1448, in _execute_child
FileNotFoundError: [Errno 2] No such file or directory: 'mix'
```

Do I need to configure anything in particular in order to get the formatter to work?