Suppot alphanumeric process ids

If process names have alphanumeric ids, `heroku ps` crashes when trying to sort them. The desired output is:

```
$ heroku ps
=== web (1X): `bundle exec rackup -s thin -p $PORT`
web.0ac72f69: up 2013/06/11 19:21:47 (~ 0s ago)
web.5c560432: up 2013/06/11 19:21:47 (~ 0s ago)
```

This change prevents processes with non-integer ids to be sorted.