Collect customers' credit card details

Previously, we were only collecting how a customer would like to pay for
their order without actually taking any payment details, which meant
that we weren't getting paid. Integrated Stripe to collect customers'
card details when they create an order.

https://trello.com/c/lhpEeBpQ
Refactoring some things
Add Stripe integration
Remove Stripe exception handling

Previously, an exception was being caught when the Stripe API returned
an error, which has not been scoped into the project yet. The exception
block has been removed from the Order controller's create action.
Add Stripe keys to test environment
Use the Travis config from suspenders
Remove test variables
Fix typo in Travis config