Encoding error on Ubuntu 14.04

I'm trying to get the waveform of a file as JSON using the following options:

``` js
var options = { scan: false,
waveformjs: '-',
'wjs-width': 34112,
'wjs-precision': 2,
'wjs-plain': true
};

waveform(myFilePath,options,function(err,buf){
var parsed = JSON.parse(buf);
// do stuff with parsed
});
```

This works fine on OS X, but running the same thing on Ubuntu 14.04 I get the following error:

```
string_decoder.js:24
throw new Error('Unknown encoding: ' + encoding);
^
Error: Unknown encoding: buffer
at assertEncoding (string_decoder.js:24:11)
at new exports.StringDecoder (string_decoder.js:30:3)
at Socket.Readable.setEncoding (_stream_readable.js:196:33)
at exports.execFile (child_process.js:672:16)
at module.exports (/home/ubuntu/animator/node_modules/waveform/index.js:22:3)
```

This is after installing `libgroove-dev`.

If i run the command directly:

```
./waveform --wjs-width 34112 --wjs-precision 4 --wjs-plain myFilePath --waveformjs -
```

it works fine. Somehow it seems like the node wrapper isn't able to correctly encode/decode the stdout buffer on Ubuntu even though it's OK on Mac. Any idea why this would be?