Improve Ergonomics of Rollout::Feature#active?

[reneklacan]

This changes signature of Rollout::Feature#active? to not require rollout instance as first argument.

When querying same feature for different users, it was required to do something like:

feature = $rollout.get(:foo)
users.each do |user|
   puts feature.active?($rollout, user)
end

This PR changes API so it's possible to just do:

feature = $rollout.get(:foo)
users.each do |user|
   puts feature.active?(user)
end

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Note: This API was never advertised but just to be sure, we just bump major version when releasing this change.