Version 1.1 doesn't work on Debian

Debian 7.7, Node 0.10.33

```
Command failed: df: unrecognized option '--output=source,size,used,avail,pcent,target,fstype'
Try `df --help' for more information.

    at ChildProcess.exithandler (child_process.js:648:15)
    at ChildProcess.emit (events.js:98:17)
    at maybeClose (child_process.js:756:16)
    at Process.ChildProcess._handle.onexit (child_process.js:823:5)
```

Version 1.0 was ok.
