Update rails, gems, use MathJax over CDN

bin/yarn

@@ -1,8 +1,8 @@
 #!/usr/bin/env ruby.exe
-VENDOR_PATH = File.expand_path('..', __dir__)
-Dir.chdir(VENDOR_PATH) do
+APP_ROOT = File.expand_path('..', __dir__)
+Dir.chdir(APP_ROOT) do
   begin
-    exec "yarnpkg #{ARGV.join(" ")}"
+    exec "yarnpkg", *ARGV
   rescue Errno::ENOENT
     $stderr.puts "Yarn executable was not detected in the system."
     $stderr.puts "Download Yarn at https://yarnpkg.com/en/docs/install"