Incompatible with rack 3

Requests aren't passing via serverless-offline due to:

```ruby
$app ||= Rack::Builder.parse_file($config['config_path'] || 'config.ru').first
#  undefined method `first' for #<Rack:...
```

which does not work for rack 3 (it returns the app object alright). The `.first` call needs to be removed.