DEV: Enable unless cops

We discussed the use of `unless` internally and decided to enforce
available rules from rubocop to restrict its most problematic uses.

spec/support/concurrency.rb

@@ -21,7 +21,7 @@ module Concurrency
         def choose(*options)
           raise DeadEnd if options.empty?
 
-          @path << [options.size, 0] unless @index < @path.size
+          @path << [options.size, 0] if @index >= @path.size
 
           pair = @path[@index]
           raise "non-determinism" unless pair[0] == options.size