Log Message:
jsoncpp: updated to 1.8.4

1.8.4:
Functional changes
* Serialize UTF-8 string with Unicode escapes
* Un-deprecate removeMember overloads, return void
* Allow Json::Value to be used in a boolean context
* Use move ctor in append()
* Add value_type to improve integration with boost

Meta-changes
* Fix meson.build to allow using jsoncpp as a subproject
* Fix some compiler warnings

Update docs
* Update for TravisCI