Stop using failing ujson as default serializer

[gghez]

ujson is no more actively maintained and specially for some critical bugs not present in json package.

one example forcing us to revert ujson usage:
ultrajson/ultrajson#325 (2 days ago, mine)
ultrajson/ultrajson#301 (same but 1 year ago and no answer)

[ziirish]

Thanks for the report.

We will address this shortly hopefully.