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 
Not sure what your approach in this gem is, but shouldn't this require be in files where this is actually needed? Otherwise this is no better than just autoloading.