Heroku healthcheck plugin install error

error.txt

$ h plugins:install https://github.com/neilmiddleton/heroku-healthcheck.git
Installing heroku-healthcheck... failed

 !    Unable to load plugin heroku-healthcheck.
 !    Search for help at: https://help.heroku.com
 !    Or report a bug at: https://github.com/heroku/heroku/issues/new

    Error:       no such file to load -- json (LoadError)
    Backtrace:   /Users/ryan/.heroku/plugins/heroku-healthcheck/init.rb:2:in `require'
                 /Users/ryan/.heroku/plugins/heroku-healthcheck/init.rb:2
                 /Users/ryan/.heroku/client/lib/heroku/plugin.rb:68:in `load'
                 /Users/ryan/.heroku/client/lib/heroku/plugin.rb:68:in `load_plugin'
                 /Users/ryan/.heroku/client/lib/heroku/command/plugins.rb:46:in `install'
                 /Users/ryan/.heroku/client/lib/heroku/helpers.rb:236:in `action'
                 /Users/ryan/.heroku/client/lib/heroku/command/plugins.rb:44:in `install'
                 /Users/ryan/.heroku/client/lib/heroku/command.rb:207:in `send'
                 /Users/ryan/.heroku/client/lib/heroku/command.rb:207:in `run'
                 /Users/ryan/.heroku/client/lib/heroku/cli.rb:28:in `start'
                 /usr/bin/heroku:25

    Command:     heroku plugins:install https://github.com/neilmiddleton/heroku-healthcheck.git
    Plugins:     external-repo-cli
                 heroku-anvil
                 heroku-healthcheck
                 heroku-orgs
                 heroku-pg-transfer
                 heroku-pipeline
                 heroku-sudo
                 heroku-td
                 heroku-tiers

    Version:     heroku-toolbelt/2.34.0 (universal-darwin12.0) ruby/1.8.7