start supporting spree 2.4 stable

Currenlty I am working on a Spree project (using spree-2.4 stable). I have tested this in my machine and it works just fine( with one small hack, https://github.com/stefansenk/spree_multi_tenant/issues/14).
```Spree::TokenizedPermission``` is no longer available in Spree, hence removed it.