Enable ES7 Transforms With react-rails

The react-rails gem adds JSX and ES6 transforms to the asset pipeline. By using .js.jsx and .es6.jsx extensions with relevant files, the asset pipeline will know to make the appropriate transformation when compiling application assets. ES7 transforms are not enabled by default, but can be configured. Add the following to the config/application.js file to allow ES7's class properties syntax:

config.react.jsx_transform_options = {
  optional: ["es7.classProperties"]
}

h/t Mike Chau