/robots.txt: enable HTTP caching.

Cache /robots.txt at the HTTP level because it rarely changes but it
gets requested by bots relatively frequently.

app/controllers/robots_controller.rb

@@ -2,5 +2,6 @@ class RobotsController < ApplicationController
   respond_to :text
 
   def index
+    expires_in 1.hour, public: true unless response.cache_control.present?
   end
 end