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.
This is quick fix to make it work. :)
You, likely, right, so check it yourself, and make PR. :)