Fixed updating of stripe donations

[bmispelon]

This error came on Sentry:

KeyError: 'subscriptions'
  File "stripe/_stripe_object.py", line 170, in __getattr__
    return self[k]
  File "stripe/_stripe_object.py", line 212, in __getitem__
    raise err
  File "stripe/_stripe_object.py", line 200, in __getitem__
    return super(StripeObject, self).__getitem__(k)

AttributeError: subscriptions
  File "django/core/handlers/exception.py", line 55, in inner
    response = get_response(request)
  File "django/core/handlers/base.py", line 197, in _get_response
    response = wrapped_callback(request, *callback_args, **callback_kwargs)
  File "django/views/decorators/cache.py", line 80, in _view_wrapper
    response = view_func(request, *args, **kwargs)
  File "fundraising/views.py", line 135, in manage_donations
    modify_donations_formset.save()
  File "django/forms/models.py", line 796, in save
    return self.save_existing_objects(commit) + self.save_new_objects(commit)
  File "django/forms/models.py", line 945, in save_existing_objects
    saved_instances.append(self.save_existing(form, obj, commit=commit))
  File "django/forms/models.py", line 773, in save_existing
    return form.save(commit=commit)
  File "fundraising/forms.py", line 164, in save
    subscription = customer.subscriptions.retrieve(donation.stripe_subscription_id)
  File "stripe/_stripe_object.py", line 172, in __getattr__
    raise AttributeError(*err.args) from err

[alexgmin]

LGTM