Strange bug with starting resque-pool

Hi,

We've run into a bug here using `resque-pool`:

```
resque-pool-0.3.0/lib/resque/pool.rb:312:in `worker_delta_for': undefined method `-' for nil:NilClass (NoMethodError)
```

I haven't been able to fully understand the issue where here's how the `config` object looks at line `312`:
```
{"ac446872-4ad9-4f69-b075-a8017aa9e049"=>1, "foo-1"=>nil}
```

The function is called for `foo-1` so we get `nil` and boom.

This PR is a naive fix.. Maybe there's a better way to deal with this issue?