feat(react.js) build with yarn; update to React 15.4.1

Author: rmosolgo

.gitignore

@@ -7,3 +7,4 @@ gemfiles/*.lock
 /vendor/react
 react-builds/node_modules
 react-builds/build
+coverage/

Rakefile

@@ -40,7 +40,7 @@ namespace :react do
   desc "Use NPM to install the JavaScript dependencies"
   task :install do
     Dir.chdir("react-builds") do
-      `npm install`
+      `yarn install`
     end
   end
 end