Gymnasium was closed

ref. https://docs.gitlab.com/ee/user/project/import/gemnasium.html#why-is-gemnasium-com-closed

Author: ttanimichi

README.md

@@ -3,7 +3,6 @@
 [![Gem](https://img.shields.io/gem/v/react-rails.svg?style=flat-square)](http://rubygems.org/gems/react-rails)
 [![npm](https://img.shields.io/npm/v/react_ujs.svg?style=flat-square)](https://www.npmjs.com/package/react_ujs)
 [![Build Status](https://img.shields.io/travis/reactjs/react-rails/master.svg?style=flat-square)](https://travis-ci.org/reactjs/react-rails)
-[![Gemnasium](https://img.shields.io/gemnasium/reactjs/react-rails.svg?style=flat-square)](https://gemnasium.com/reactjs/react-rails)
 [![Code Climate](https://img.shields.io/codeclimate/github/reactjs/react-rails.svg?style=flat-square)](https://codeclimate.com/github/reactjs/react-rails)