add attr_accessible that is used by seed

Ran `spree:install` and the seed failed due to mass-assignment of fields in `Spree::Country`. Simply added the `attr_accessible` to `Spree::Country`.

---
Output from the exec:

    bundle exec rails g spree:install
          create  config/initializers/spree.rb
          create  config/spree.yml
          remove  public/index.html
          append  public/robots.txt
          create  app/assets/javascripts/store
          create  app/assets/javascripts/admin
          create  app/assets/stylesheets/store
          create  app/assets/stylesheets/admin
          create  app/assets/images/store
          create  app/assets/images/admin
          create  app/assets/javascripts/store/all.js
          create  app/assets/stylesheets/store/all.css
          create  app/assets/javascripts/admin/all.js
          create  app/assets/stylesheets/admin/all.css
          create  app/overrides
          append  db/seeds.rb
         copying  migrations
        creating  database
         running  migrations
         loading  seed data
            rake  db:seed 
    loading ruby /home/zinger/.rbenv/versions/1.9.3-p392/lib/ruby/gems/1.9.1/bundler/gems/spree-4059a35c41e4/core/db/default/spree/countries.rb
    rake aborted!
    Validation failed: Name can't be blank, ISO Name can't be blank
    /home/tmp/blah/db/seeds.rb:10:in `<top (required)>'
    Tasks: TOP => db:load_dir
    (See full trace by running task with --trace)
         loading  sample data
          insert  config/routes.rb
    **************************************************
    We added the following line to your application's config/routes.rb file:
     
        mount Spree::Core::Engine, :at => '/'
    **************************************************
    Spree has been installed successfully. You're all ready to go!
     
    Enjoy!