This means you’re using a RubyGems version older than
1.3.3, which doesn’t support gem plugins. Please upgrade to
the latest by running
gem update --system.
If you haven’t gotten it, feel free to create a new Issue here on Tender and we'll get you sorted out.
Click on the “download” link to the right of the gem’s name on that gem’s page.
If you want to set up your own Gemcutter instance, please check out the Standup your own Gemcutter guide. If you want to hack on the project and add your own contributions, please see the Contribution Guidelines to get started.