Rubyracer does not work on ruby, so add it to Gemfile with :ruby platform only

railties/lib/rails/generators/app_base.rb

@@ -225,7 +225,7 @@ def javascript_runtime_gemfile_entry
         if defined?(JRUBY_VERSION)
           "gem 'therubyrhino'\n"
         else
-          "# gem 'therubyracer'\n"
+          "# gem 'therubyracer', :platform => :ruby\n"
         end
       end