replaced the pulled code with proper gem-reference

The railtie now automatically makes sure, that the middleware is used,
and the included gem better respects the copyright of the used solution.

lib/request_store/middleware.rb

@@ -1,20 +0,0 @@
-module RequestStore
-  class Middleware
-    def initialize(app)
-      @app = app
-    end
-
-    def call(env)
-      RequestStore.clear!
-      @app.call(env)
-    end
-  end
-
-  def self.store
-    Thread.current[:request_store] ||= {}
-  end
-
-  def self.clear!
-    Thread.current[:request_store] = {}
-  end
-end