PERF: Add missing index on `EmailLog#bounced`.

* Also remove index on `user_id_and_created_at` which
  isn't being used alot.

db/migrate/20180803085321_add_index_email_logs_on_bounced.rb

@@ -0,0 +1,6 @@
+class AddIndexEmailLogsOnBounced < ActiveRecord::Migration[5.2]
+  def change
+    add_index :email_logs, :bounced
+    remove_index :email_logs, [:user_id, :created_at]
+  end
+end