In the spec, I'm testing both methods of group defining together:
```ruby
@rollout.define_group(:fivesonly)     { |user|    user.id == 5 }
@rollout.define_group(:fives_id_only) { |user_id| user_id == 5 }
```

Having the mixed lambas results in a `NoMethod` exception occur when calling the `{ |user|    user.id == 5 }` lamda is called against a pure id (`undefined method 'id' for 5:Fixnum`)... I thought about separating the specs, but figured that rescuing it and returning false would make more sense, and help anyone transitioning between the two styles of group defining/checking; basically ends up just guarding against client code... which likely example of is tested in the specs.

If you have a preference for how you'd like it changed, I can adjust accordingly :)