Fix configuration initializer

The README recommends to use the following initializer:

```ruby
TBK::configure do |config|
  config.commerce_id YOUR_COMMERCE_ID
  config.commerce_key YOUR_RSA_KEY
end
```

However, it triggers the following error:
```
... /config/initializers/tbk.rb:5:in `<top (required)>': undefined method `configure' for TBK:Module (NoMethodError)
```

On the other hand, in `lib/tbk/config.rb` the recommended initializer calls `TBK.configure` instead of `TBK::configure`. AS of, that call triggers the same error.