Update Stripe Version

[westbl]

We're currently running version 2015-03-24; the latest version is 2019-05-16. Not 100% sure this is necessary, but probably should be done at some point in the future.

Documentation: https://stripe.com/docs/upgrades#how-can-i-upgrade-my-api

[ppannuto]

I looked into this a little, so there's a few things:

1.) We aren't using the Stripe API directly, rather we're using their checkout.js feature, which stays up to date because it's loaded directly from stripe.

1a.) We did briefly explore writing our own applet, which is the last time we would've hit that API, and why the dashboard reports such an old API version. That's not meaningful in practice, however.

2.) In April 2019, Stripe redid checkout

2a.) We can keep using what we have for the forseeable future

The legacy version of Checkout can still be used to accept payments, but is no longer in active development and does not support the upcoming European Strong Customer Authentication regulation.

2b.) They have a migration guide, but it's going to take a full day or so to do this correctly, as our flow isn't really supported any more. We have to create a new callback endpoint and credit user accounts a a post-purchase fulfillment step. The other option is the move over to Stripe elements, which is more like building a custom applet. I'm not sure which of these is better yet.

Takeaway: We will have to upgrade eventually, but it looks sadly non-trivial