Remove unmatched uv_unref() causing segfault

Fixes https://github.com/justinlatimer/node-midi/issues/16

The tests seem to hang but I'm not sure if that's normal:
```
amorton@pavlov:~/projects/node-midi% npm test

> midi@0.7.0 test /Users/amorton/projects/node-midi
> node test/virtual-loopback-test-automated.js

Enumerating inputs
Input found: MidiKeys
Input found: node-midi Virtual Output
Opening node-midi Virtual Output
Enumerating outputs
Output found: MidiKeys
Opening node-midi Virtual Input
Sending message
Virtual input recieved m:144,23,81 d:0
Input recieved m:144,33,81 d:0
Sending message
Virtual input recieved m:144,23,81 d:1.00095084
Input recieved m:144,33,81 d:1.000284769
Sending message
Virtual input recieved m:144,23,81 d:1.00113951
Input recieved m:144,33,81 d:1.00112985
Sending message
Virtual input recieved m:144,23,81 d:1.002199748
Input recieved m:144,33,81 d:1.002168173
Sending message
Virtual input recieved m:144,23,81 d:1.002636719
Input recieved m:144,33,81 d:1.002560755
Sending message
Virtual input recieved m:144,23,81 d:1.0018766590000001
Input recieved m:144,33,81 d:1.001880571
Sending message
Virtual input recieved m:144,23,81 d:1.001113335
Input recieved m:144,33,81 d:1.001847098
Sending message
Virtual input recieved m:144,23,81 d:1.001633942
Input recieved m:144,33,81 d:1.000983079
Sending message
Virtual input recieved m:144,23,81 d:1.000306951
Input recieved m:144,33,81 d:1.000308239

```

I'll try building it on 0.8.x and see what the tests do there.