use the DjangoJSONEncoder when dumping exhibit data

As pointed out by @zepheiryan, loading data with decimal values breaks the builder.  When the django_extensions JSONField switched to python's json module from simplejson, we followed suit and everything was fine.  At some point since then, they switched over to using django.core.serializers.json.DjangoJSONEncoder on save. This patch uses that in the exhibit views as well.