Bug#886758: New version 1.8.4 available

[Yangfl]

Package: libjsoncpp
Version: 1.7.4-3

The current Debian version is outdated. Since I'm packing
https://github.com/avast-tl/retdec-config which require 1.8.4, it
would be nice to update it to the current upstream one.

Thanks,