Fix error: undefined local variable or method `user_model' for CASServer...

Hello!
This PR contains fix for error:
```
undefined local variable or method `user_model' for CASServer::Authenticators::SQLEncrypted:Class (NameError)
```
See #219 for more.
I think this should actually be:

```ruby
user_model = user_models[options[:auth_index]]
user_model.__send__(:include, EncryptedPassword)
```