Had some trouble doing the normal composer install so I went with the manual install noted here https://github.com/drush-ops/drush

Basically I downloaded and unzipped the repo to /usr/local/sbin. Then I did a symlink to /usr/bin

cd /usr/bin
sudo ln -s /usr/local/sbin/drush/drush .

Then in /usr/local/sbin/drush I ran

sudo composer install