use timestamp instead of created_at

app/admin/crash_dumps.rb

@@ -1,12 +1,12 @@
 ActiveAdmin.register JamRuby::CrashDump, :as => 'Crash Dump' do 
   # Note: a lame thing is it's not obvious how to make it search on email instead of user_id.
-  filter :created_at
+  filter :timestamp
   filter :user_email, :as => :string
   filter :client_id
 
   index do
     column "Timestamp" do |post|
-      post.created_at.strftime('%b %d %Y, %H:%M')
+      post.timestamp.strftime('%b %d %Y, %H:%M')
     end
     column "Client Type", :client_type
     column "Dump URL" do |post|