Upgrade formpack to fix #2057

[jnm]

Fixes #2057. ⚠️ references a commit from an unmerged PR on formpack: kobotoolbox/formpack#180

This is now running on https://kf.master.kbtdev.org/. To test:

1. Create a blank form;
2. Add a "Select One" question;
3. Drag "Option 2" to the top;
4. Click the preview icon.

The fix only works for new AssetSnapshots. That means that trying to preview outside the form builder, on a form that has not been saved since deploying this fix, will still fail with argument of type 'int' is not iterable. To clean up the mess, we will need to run this:

AssetSnapshot.objects.filter(xml='', details__contains='''"argument of type 'int' is not iterable"''').delete()