When Capistrano3 does mkdir, permission denied

When I did bundle exec cap production deploy, i got error messages like mkdir: cannot create directory /usr/share/nginx/www/appname/releases/20131216155457: Permission denied

Capistrano3 doesn't have set use_sudo and default_run_options. I read that Passwordless sudo is better, so I did visudo username ALL=NOPASSWD:ALL, but still I got the same errors.

set :application, 'appname.com'
set :repo_url, 'git@bitbucket.org:user/myapp.git'
set :user, 'username' 
set :branch, 'master'


set :deploy_to, '/usr/share/nginx/www/appname'
set :keep_releases, 5

namespace :deploy do

 desc 'Restart application'
 task :restart do
   on roles(:app), in: :sequence, wait: 5 do
   end
 end

 after :restart, :clear_cache do
   on roles(:web), in: :groups, limit: 3, wait: 10 do
   end
 end

 after :finishing, 'deploy:cleanup'

end

Does anyone give me any advice around this problem? Thank you for your help.