Use Dalli::Protocol::Binary to address deprecataion warning

```ruby
Dalli::Server is deprecated, use Dalli::Protocol::Binary instead
```

Refer petergoldstein/dalli#760

Author: yahonda

activesupport/test/cache/stores/mem_cache_store_test.rb

@@ -19,7 +19,7 @@ def get(key, options = {})
     end
   end
 
-  class UnavailableDalliServer < Dalli::Server
+  class UnavailableDalliServer < Dalli::Protocol::Binary
     def alive?
       false
     end