Update JSON gem to <2.0

When running Fontcustom with Ruby 3.x, am getting this error.

`.rvm/gems/ruby-3.0.0/gems/json-1.8.6/lib/json/common.rb:155:in `initialize': wrong number of arguments (given 2, expected 1) (ArgumentError)`

If I'm understanding it correctly, this is happening because the older JSON gem isn't compatible with current Ruby. Should this be modified to keep the build up-to-date?