fix exec path for Procfile.dev (rails#125)

Author: d6rkaiz

lib/install/Procfile.dev

@@ -1,2 +1,2 @@
 web: bin/rails server -p 3000
-css: rails tailwindcss:watch
+css: bin/rails tailwindcss:watch

lib/install/tailwindcss.rb

@@ -35,7 +35,7 @@
 end
 
 if Rails.root.join("Procfile.dev").exist?
-  append_to_file "Procfile.dev", "css: rails tailwindcss:watch\n"
+  append_to_file "Procfile.dev", "css: bin/rails tailwindcss:watch\n"
 else
   say "Add default Procfile.dev"
   copy_file "#{__dir__}/Procfile.dev", "Procfile.dev"