Remove redundant arguments in store test helper

The store now uses $MEMCACHE_SERVERS and 127.0.0.1:11211 as fallbacks,
so there is no need to specify them them again.

activesupport/test/cache/stores/mem_cache_store_test.rb

@@ -202,7 +202,7 @@ def random_string(length)
     end
 
     def store
-      [:mem_cache_store, ENV["MEMCACHE_SERVERS"] || "localhost:11211"]
+      [:mem_cache_store]
     end
 
     def emulating_latency