Composer, Drush & Drupal 8.4.0
On my system (Debian 8.3) I was using Drush 9.0-dev (I know, you should never use a dev version in production but I did this before I knew any better). To make a long story short I tried to change my drush version to the latest (at this time 8.1.13). I researched what I needed to do to make this change but nothing worked. I ended up having to remove (rename the syslink) and then installed the version I wanted with composer.
To remove (rename) the old version of drush:
1) rename /usr/bin/drush to /usr/bin/drush.v9
2) rename directory /usr/local/src/drush to /usr/local/src/drush.v9-dev
#optional but if you don't rename it you can use
Drush 9.0-dev by issuing the command drush.v9
To install Drush 8.1.13:
1) composer global require drush/drush:8.1.13
2) export PATH="$HOME/.composer/vendor/bin:$PATH"
3) source ~/.bash_profile