Set a fixed API version

[driesvints]

There have been numerous issues with Stripe API updates in the past. Here's some:

• [Stripe API Version Incompatbility] 2018-08-23 no longer allows at_period_end to be set when cancelling #551
• [9.0] Update invoice date method to work with latest '2019-03-14' stripe api. #617

The problem with this is that Cashier currently tries to be compatible with numerous API updates. We do, however started testing against a specific version (usually the latest one) since #603

But because we don't enforce this version through the SDK, Stripe can still release updates and people can update the API version themselves which will cause breakages like #617.

A possible solution for this is to enforce a version through the SDK so the library is always compatible with the specific version of the API that it uses. The problem with this however is that if users are extending Cashier and using API specific functionality will see their apps break if we upgrade to a new version.

I do, however, believe this is the correct way forward. This would mean that we need to set a specific version for Cashier through the SDK. Making this configurable doesn't makes any sense since if someone would change this to a version that has breakages for Cashier, Cashier itself wouldn't be usable.

By setting a specific version this would prevent breakages like above and would allow us to adopt our own upgrade path as we see fit. This would mean we could only adopt breaking API version changes in new minor versions which SemVer allows for. People extending the library however, need to be aware that this can happen and need to be aware to not upgrade blindly. SemVer allows for us to upgrade dependencies as we see fit as long as we don't alter the public API of Cashier itself: https://semver.org/#what-should-i-do-if-i-update-my-own-dependencies-without-changing-the-public-api

Because of the above reason we may adopt this as soon as the next minor version of Cashier. But for the sake of explicitness and the fact that Cashier handles very sensitive data it might be better to make this change in the next major version and add a general note to the docs about Stripe versions and upgrading minor versions when a Stripe API update to Cashier was performed.