Update rails, gems, use MathJax over CDN

bin/update

@@ -1,10 +1,9 @@
 #!/usr/bin/env ruby.exe
-require 'pathname'
 require 'fileutils'
 include FileUtils
 
 # path to your application root.
-APP_ROOT = Pathname.new File.expand_path('../../', __FILE__)
+APP_ROOT = File.expand_path('..', __dir__)
 
 def system!(*args)
   system(*args) || abort("\n== Command #{args} failed ==")
@@ -18,6 +17,9 @@ chdir APP_ROOT do
   system! 'gem install bundler --conservative'
   system('bundle check') || system!('bundle install')
 
+  # Install JavaScript dependencies if using Yarn
+  # system('bin/yarn')
+
   puts "\n== Updating database =="
   system! 'bin/rails db:migrate'