🐛 Fixed comp plans for members w/ active subscriptions

refs https://github.com/TryGhost/Team/issues/475

The subscription object here is a database model rather than an
ISubscription from the stripe library, and we need to 1) use the `get`
method to read attributes and 2) read the `subscription_id` attribute,
as the `id` is our internal one.

ghost/members-api/lib/repositories/member/index.js

@@ -310,7 +310,7 @@ module.exports = class MemberRepository {
             // NOTE: we should only ever have 1 active subscription, but just in case there is more update is done on all of them
             for (const subscription of activeSubscriptions) {
                 const updatedSubscription = await this._stripeAPIService.changeSubscriptionPlan(
-                    subscription.id,
+                    subscription.get('subscription_id'),
                     complimentaryPlan.id
                 );