Fixed stripe API usage in update_card view

Refs #1912

Author: bmispelon

fundraising/views.py

@@ -156,13 +156,10 @@ def manage_donations(request, hero):
 def update_card(request):
     donation = get_object_or_404(Donation, id=request.POST["donation_id"])
     try:
-        customer = stripe.Customer.retrieve(
-            donation.stripe_customer_id, expand=["subscriptions"]
+        stripe.Customer.modify(
+            donation.stripe_customer_id,
+            source=request.POST["stripe_token"],
         )
-        token = stripe.Token.retrieve(request.POST["stripe_token"])
-        subscription = customer.subscriptions.retrieve(donation.stripe_subscription_id)
-        subscription.default_source = token.card.id
-        subscription.save()
     except stripe.error.StripeError as e:
         data = {"success": False, "error": str(e)}
     else: