Upgrade the Engines plugin to get the bugfix for Rails::Plugin::GemLocator

This update will allow Rails Engines to be installed as gems.  For example:

    # config/environment.rb
    Rails::Initializer.run do |config|
      ...
      config.gem 'timesheet_plugin'
    end

Will load the timesheet_plugins's RubyGem and allow it to work as if it was
installed to vendor/plugins.

git-svn-id: svn+ssh://rubyforge.org/var/svn/redmine/trunk@2904 e93f8b46-1217-0410-a6f0-8f06a7374b81

vendor/plugins/engines/generators/plugin_migration/plugin_migration_generator.rb

@@ -48,7 +48,7 @@ class PluginMigrationGenerator < Rails::Generator::Base
         end
       end
       
-      @plugins_to_migrate.reject! { |p| p.latest_migration.nil? }
+      @plugins_to_migrate.reject! { |p| !p.respond_to?(:latest_migration) || p.latest_migration.nil? }
       
       # Then find the current versions from the database    
       @current_versions = {}