Remove warning (#167)

* Remove warning

```
`Redis#exists(key)` will return an Integer in redis-rb 4.3. `exists?` returns a boolean, you should use it instead. To opt-in to the new behavior now you can set Redis.exists_returns_integer =  true. To disable this message and keep the current (boolean) behaviour of 'exists' you can set `Redis.exists_returns_integer = false`, but this option will be removed in 5.0.
```

* Use alternative solution

Author: emancu

lib/ldclient-rb/impl/integrations/redis_impl.rb

@@ -115,7 +115,9 @@ def upsert_internal(kind, new_item)
           end
 
           def initialized_internal?
-            with_connection { |redis| redis.exists(inited_key) }
+            with_connection do |redis|
+              redis.respond_to?(:exists?) ? redis.exists?(inited_key) : redis.exists(inited_key)
+            end
           end
 
           def stop