jsoncpp: 1.8.0 -> 1.8.4

Semi-automatic update. These checks were performed:

- built on NixOS
- found 1.8.4 with grep in /nix/store/y4i1yg5dyczsifajqcmws9hv5aqy502k-jsoncpp-1.8.4
- found 1.8.4 in filename of file in /nix/store/y4i1yg5dyczsifajqcmws9hv5aqy502k-jsoncpp-1.8.4

cc "@ttuegel @cpages"