Remove deprecated `host` Excon option from dynamodb

This addresses an excon warning:

```
[excon][WARNING] Invalid Excon request keys: :host
/Users/pedro/.rvm/gems/ruby-1.9.3-p448/gems/excon-0.32.1/lib/excon/connection.rb:389:in `validate_params'
/Users/pedro/.rvm/gems/ruby-1.9.3-p448/gems/excon-0.32.1/lib/excon/connection.rb:225:in `request'
```

Thanks!