Fix for LOGSTASH-1920

`bin/logstash` breaks if CDPATH env var is set causing cd to emit dir name:

```
$ logstash/bin/logstash --help
logstash/bin/logstash: line 22: .: /home/tim/trunk/comp/support_logstash/logstash: is a directory
logstash/bin/logstash: line 24: setup: command not found
logstash/bin/logstash: line 31: exec: -I: invalid option
```

This fixes it by sending the output to `/dev/null`