Lazy require memcache for session middleware

actionpack/lib/action_dispatch/middleware/session/mem_cache_store.rb

@@ -1,11 +1,9 @@
-require "active_support/core_ext/kernel/requires"
-begin
-  require_library_or_gem 'memcache'
-
 module ActionDispatch
   module Session
     class MemCacheStore < AbstractStore
       def initialize(app, options = {})
+        require 'memcache'
+
         # Support old :expires option
         options[:expire_after] ||= options[:expires]
 
@@ -47,6 +45,3 @@ begin
     end
   end
 end
-rescue LoadError
-  # MemCache wasn't available so neither can the store be
-end