Fix configuring third-party cache stores such as ActiveSupport::Cache::RedisStore

Broken in 8da30ad.

activesupport/lib/active_support/cache.rb

@@ -101,7 +101,7 @@ module ActiveSupport
         # Obtains the specified cache store class, given the name of the +store+.
         # Raises an error when the store class cannot be found.
         def retrieve_store_class(store)
-          require_relative "cache/#{store}"
+          require "active_support/cache/#{store}"
         rescue LoadError => e
           raise "Could not find cache store adapter for #{store} (#{e})"
         else