create db-path

so you don't get

```
events.js:72
        throw er; // Unhandled 'error' event
              ^
OpenError: IO error: /home/ubuntu/.npmd/db/LOCK: No such file or directory
    at /usr/local/lib/node_modules/npmd/node_modules/levelup/lib/levelup.js:114:34
```

on the first run