Update univalue to latest release (1.0.4)

Summary:
Bring univalue library up to the the following commit:

commit 88967f658663654a67f1cda93ca6f99de1f7c91f (univalue/master)
Author: Jeff Garzik <jgarzik@pobox.com>
Date:   Fri Aug 31 21:32:32 2018 -0400

    Version 1.0.4

Test Plan:

make check

Reviewers: #bitcoin_abc, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Subscribers: teamcity

Differential Revision: https://reviews.bitcoinabc.org/D1989