Add amount paid

Amount paid colum is added to ticket purchase to keep the record of money paid by the user.
It is added to physical_ticket#index to show admin, price paid by each user for each ticket.

db/migrate/20170924190528_add_amount_paid_to_ticket_purchases.rb

@@ -0,0 +1,5 @@
+class AddAmountPaidToTicketPurchases < ActiveRecord::Migration
+  def change
+    add_column :ticket_purchases, :amount_paid, :float, default: 0
+  end
+end