Fix Capistrano Tasks to Use Stage

This pull request references issue #163 where capistrano tasks fail to create sitemaps because it cannot determine the environment. It seems that capistrano 3 uses stages to determine the RAILS_ENV in addition to what the environment is. I added code identical to how Capistrano itself determines its environment: https://github.com/capistrano/rails/blob/master/lib/capistrano/tasks/set_rails_env.rake

I tested the change on my setup which uses Capistrano 3 and the master branch of sitemap_generator and this seems to correct the problem.