Add missing gem json

This will fix :

~/callme$ bundle exec thin --max-persistent-conns 1024 --timeout 0 -R config.ru -p 8080 start
The source :rubygems is deprecated because HTTP requests are insecure.
Please change your source to 'https://rubygems.org' if possible, or 'http://rubygems.org' if not.
./application.rb:6:in `require': no such file to load -- json (LoadError)
        from ./application.rb:6
        from config.ru:1:in `require'
        from config.ru:1
        from /var/lib/gems/1.8/gems/rack-1.3.6/lib/rack/builder.rb:51:in `instance_eval'
        from /var/lib/gems/1.8/gems/rack-1.3.6/lib/rack/builder.rb:51:in `initialize'
        from config.ru:1:in `new'
        from config.ru:1