FIX: 99999 is not a valid TCP port

Future versions of redis will validate this port number causing the tests
relying on this to fail with:

```
Redis::CommandError:
ERR Invalid master port
```

Also change from an IPv4 address that might feasibly be in use to an IPv6
random ULA address that almost *certainly* won't be.

spec/lib/rate_limiter_spec.rb

@@ -105,7 +105,8 @@ describe RateLimiter do
 
     context 'handles readonly' do
       before do
-        Discourse.redis.without_namespace.slaveof '10.0.0.1', '99999'
+        # random IP address in the ULA range that does not exist
+        Discourse.redis.without_namespace.slaveof 'fdec:3f5d:d0b7:4c4b:472b:636a:4370:7ac5', '49999'
       end
 
       after do