connect() to unix:/var/run/php5-fpm.sock failed (13: Permission denied) while connecting to upstream

Abrir:

/etc/php5/fpm/pool.d/www.conf

Asignar el usuario adecuado:

listen.owner = www-data
listen.group = www-data
listen.mode = 0660

Reiniciar el servicio:

$ sudo service php5-fpm restart