Fixed Procfile.dev to remove unnecessary empty line and terminated with a newline (#23)

Author: okonomi

lib/install/install.rb

@@ -38,7 +38,7 @@
 end
 
 if Rails.root.join("Procfile.dev").exist?
-  append_to_file "Procfile.dev", "\ncss: yarn build:css --watch"
+  append_to_file "Procfile.dev", "css: yarn build:css --watch\n"
 else
   say "Add default Procfile.dev"
   copy_file "#{__dir__}/Procfile.dev", "Procfile.dev"