Define cache expire times for JSON requests

JSON requests are very expensive. Make sure they are cached for long
enough.

app/controllers/json_controller.rb

@@ -1,21 +1,25 @@
 class JsonController < ApplicationController
   # JSON information on a single package
   def package
+    expires_in 1.hours, public: true
     @p = Package.find_by_name! params[:name]
   end
 
   # JSON information on all packages
   def packages
+    expires_in 1.days, public: true
     @p = Package.all
   end
 
   # JSON information on all screenshots
   def screenshots
+    expires_in 1.days, public: true
     @s = Screenshot.includes(:package)
   end
 
   # JSON list of packages that do not have screenshots
   def packages_without_screenshots
+    expires_in 1.hours, public: true
     @p = Package.without_screenshots.all
   end
 end