DEV: properly require csv dependency

csv dependency is not default loaded, explicitly ask for it when we start
parsing csv files.

app/controllers/invites_controller.rb

@@ -168,6 +168,8 @@ class InvitesController < ApplicationController
   end
 
   def upload_csv
+    require 'csv'
+
     guardian.ensure_can_bulk_invite_to_forum!(current_user)
 
     hijack do