Update Ruby

Previously, the site was running on Ruby 2.1.2, which opened us up to
potential security and performance issues. Updated Ruby to 2.1.5

https://trello.com/c/E2leIVIy
Tidy up new order validations

Previously, the client-side validations on the new order form were too
verbose, which was causing confusion for our customers. Updated the
validations to read better by using the Stripe library.

https://trello.com/c/TSWVInJ3