refactor payment#purchase method, improve environment variable names

db/migrate/20160606040848_create_payments.rb

@@ -7,8 +7,8 @@ class CreatePayments < ActiveRecord::Migration
       t.decimal :amount, precision: 12, scale: 3
       t.string :authorization_code
       t.integer :status, default: 0
-      t.integer :user_id
-      t.integer :conference_id
+      t.integer :user_id, null: false
+      t.integer :conference_id, null: false
       t.datetime :created_at
       t.datetime :updated_at