Apparently, the drush devs have killed off 5.9.0...

No 5.x branches listed on https://packagist.org/packages/drush/drush...

tests/scripts/travis_setup.sh

@@ -19,7 +19,7 @@ pear channel-discover pear.phpqatools.org
 pear channel-discover pear.netpirates.net
 pear install pear/PHP_CodeSniffer-1.4.8
 pear install pear.phpunit.de/phpcpd
-composer global require drush/drush:5.9.0
+composer global require drush/drush:6.x
 phpenv rehash
 drush dl --yes drupal
 cd drupal-*