preserve Rails::Plugin::GemLocator in default_plugin_locators

boot.rb

@@ -10,7 +10,7 @@
 # initialize Rails::Configuration with our own default values to spare users 
 # some hassle with the installation and keep the environment cleaner
 
-{ :default_plugin_locators => [Engines::Plugin::FileSystemLocator],
+{ :default_plugin_locators => (defined?(Gem) ? [Rails::Plugin::GemLocator] : []).push(Engines::Plugin::FileSystemLocator),
   :default_plugin_loader => Engines::Plugin::Loader,
   :default_plugins => [:engines, :all] }.each do |name, default|    
   Rails::Configuration.send(:define_method, name) { default }