Set the default rollout strategy to round robin

This is required to make the multivariate concept native, while also keeps the behavior of “first” in that it will return the first variant if there’s only one (or if there’s no cache). This is required to make our documentation within gitlab accurate.

I'm not going to bother including it 0.5.0 as we can simply configure it to be used by default there, but did realized it would be an issue for everyone. When I did this I also realized that it doesn't work without caching enabled, so went ahead and "fixed?" that. Though it should probably warn or something, because it would be a surprising detail if you didn't understand it.