Allow asking for any variant on an ab_test

If ab config changes variant configuration for an ab test,
ab lib used to throw errors if `ab_test.some_variant?` code
was invoked. Now it returns false.

Java/Objective-C implementations seem to behave in similar way.

cc @mmozuras 
If you've decided to do it this way, there also must be a way to detect when non-existing variants are used.