show error message when vendor directory is missing

see #336
leave minimum stability to stable
remove unnecessary loading of PHPUnit/Autoload.php
don't check if the cache directory exists

`core download` takes care of it
Merge pull request #341 from wp-cli/fix-composer

require_once(Symfony/Component/Yaml/autoloader.php): failed to open stream
install composer dependencies in local-build too

see #342
make local-build embedable into .bashrc again

see #342