move ActiveRecord dependency into ActiveRecordTicketStore

The usage of `ActiveRecord::SessionStore.session_class.find …` is kind of bothering me, as I've implemented a RedisTicketStore and I had to overwrite an additional method to achieve that.

The `AbstractTicketStore` should, as the name implies, not be coupled to any storage engine at all. Thus, I've added a method named `destroy_session_with_session_id` which needs to be implemented per TicketStore.

With this change it's quite easy to enable single sign out using redis-store and no shared tmp sessions directory.

The tests are not green yet but hopefully I can fix this tomorrow.

If you like I'd create a second pr after this one which contains a RedisTicketStore implementation, as it eased the single sign out integration for me.

thanks you :)