Update api_version to 2018-05-21

[snewcomer]

• Products no longer have SKU lists embedded.
  Never assumed this: - [ ] Major - The id field of invoice line items of type=subscription no longer can be interpreted as a subscription ID, but instead is a unique invoice line item ID. It can be used for pagination.
  Not applicable: - [ ] Coupon, SKU, customer, product and plan ids may only contain alphanumeric and _- characters on creation.
  Not applicable: - [ ] Major - When creating or updating subscriptions, the default value of trial_from_plan is now false, meaning that a subscription will not automatically inherit a plan’s trial_period_days. We recommend setting an explicit trial per subscription instead of setting trials on plans.
  Not applicable: - [ ] When changing the plan on a subscription to a new plan with a trial (together with trial_from_plan=true), the new plan’s full trial period will be added to the subscription, without subtracting already-used time from previous trials.

Ref - https://stripe.com/docs/upgrades#api-changelog

[coveralls]

Coverage remained the same at 83.473% when pulling ec79786 on snewcomer:sn/2018-05-21 into db5e873 on code-corps:master.