Also initialize ViewContext for rails runner (#739)

Initialize Draper's ViewContext when using the runner command in a
similar way to using the console command. Without this code run by the
rails runner command (e.g. when processing commands from a message
queue) will not have a properly initialized ViewContext, and
consequently the helpers will not be initialized.

lib/draper/railtie.rb

@@ -55,6 +55,12 @@ module Draper
       Draper::ViewContext.build
     end
 
+    runner do
+      require 'action_controller/test_case'
+      ApplicationController.new.view_context
+      Draper::ViewContext.build
+    end
+
     rake_tasks do
       Dir[File.join(File.dirname(__FILE__),'tasks/*.rake')].each { |f| load f }
     end