config: disable Redis by default.

Fixes Docker containers and development installs that don't have Redis
installed from throwing errors about failing to connect to Redis.

Downstream boorus who do use Redis will need to uncomment this line or
set `redis_url` manually in their config to enable Redis again.

config/danbooru_default_config.rb

@@ -562,8 +562,11 @@ module Danbooru
     def recommender_server
     end
 
+    # Uncomment to enable the Redis cache store. Caching is optional for
+    # small boorus but highly recommended for large multi-user boorus. Redis is
+    # required to enable saved searches.
     def redis_url
-      "redis://localhost:6379"
+      # "redis://localhost:6379"
     end
   end