Fix Procfile.dev to terminated with a newline (#27)

Author: okonomi

lib/install/install.rb

@@ -32,7 +32,7 @@
 end
 
 if Rails.root.join("Procfile.dev").exist?
-  append_to_file "Procfile.dev", "js: yarn build --watch"
+  append_to_file "Procfile.dev", "js: yarn build --watch\n"
 else
   say "Add default Procfile.dev"
   copy_file "#{__dir__}/Procfile.dev", "Procfile.dev"