Increasing the number of open files by docker process.

With the more extensive/intensive use of docker on day-to-day activity, this error might occur:

```bash
$ tail -f /var/log/docker.log
2014/07/02 20:40:15 http: Accept error: accept unix /var/run/docker.sock: too many open files; retrying in 1s
2014/07/02 20:40:16 http: Accept error: accept tcp [::]:2375: too many open files; retrying in 1s
```

This pull request has the goal of increasing docker's open file limit, avoiding this error.
you are correct, I made the change