Duplicates in features list

Thanks for rollout. I've been using it for a while now and it's great. Ran into an issue playing with 2.0.0rc.

Any action that calls save (activate, activate_user, deactivate_user, etc) will add a duplicate to the features array.

Here's a patch. I saw on the rewrite branch that you are using a set instead, but this fixes it on the master branch.