Create public folder if it doesn't exist already

Rakefile

 task 'build' do
   require 'sassc'
 
+  unless Dir.exist?('./web/public')
+    puts 'Creating web/public/'
+    Dir.mkdir('./web/public')
+  end
+
   scss = File.read('./web/assets/style.scss')
   css = SassC::Engine.new(scss, style: :compressed).render
   puts 'Writing web/public/style.css'