Fixed which output at each new shell creation

After #2619, I'm seeing the following each time I open a new shell:

```
/usr/bin/which: no chruby_prompt_info in (/usr/lib64/qt-3.3/bin:/usr/local/bin:/usr/bin:/bin:/home/daniel/bin:/usr/local/sbin:/usr/sbin)
```

That change removed a redirect to /dev/null that had been suppressing this output. I simply re-added that redirect.