I've looked through all similar issues reported but none of them solves my problem. I always install drush with composer and installing 6.4.0 is working just fine, but when I install dev-master then I get the error
Unable to load autoload.php. Drush now requires Composer in order to install its dependencies and autoload classes. Please see README.md
Composer is at the latest version.
When I go to ~/.composer I never get a ./vendor/ directory as suggested by answers to other questions.
My environment is Ubuntu 14.04 and I'm using PHP 5.5 - any idea what else could be wrong?