Avoid RedisCacheStore#increment on Rails 6+ (#597)

Rails has handled increment+expires_in since
6.0 (9d5b02ec50),
and more recent versions add pipeline optimizations on top of that.

lib/rack/attack/store_proxy/redis_cache_store_proxy.rb

@@ -10,8 +10,9 @@ module Rack
           store.class.name == "ActiveSupport::Cache::RedisCacheStore"
         end
 
+        if defined?(::ActiveSupport) && ::ActiveSupport::VERSION::MAJOR < 6
           def increment(name, amount = 1, **options)
-          # RedisCacheStore#increment ignores options[:expires_in].
+            # RedisCacheStore#increment ignores options[:expires_in] in versions prior to 6.
             #
             # So in order to workaround this we use RedisCacheStore#write (which sets expiration) to initialize
             # the counter. After that we continue using the original RedisCacheStore#increment.
@@ -23,6 +24,7 @@ module Rack
               super
             end
           end
+        end
 
         def read(name, options = {})
           super(name, options.merge!(raw: true))