MonkeyPatch: don't write feature if it already exists.

Always writing is causing errors when you're using Rollout in a case
where you want certain parts to only read from slaves and other parts to
only write to your master.

As we're overwriting the standard Rollout.active? class, checking for
the active state in a read-only slave would cause a Redis error. Only
writing this feature when it doesn't exist resolves this issue.

See https://github.com/jrallison/rollout_ui/pull/28