Install drush with composer

Cf: https://www.drupal.org/docs/8/update/update-procedure-in-drupal-8
# Install composer using apt-get:
sudo apt-get install composer

# Install latest drush (e.g. 9.x) using composer (2018-4-4) (not recommanded!)
# composer global require drush/drush

# Or, just install drush 8 using composer (2018-4-4)
# because drush 9.x cannot use ups (recommanded)
composer global require drush/drush:~8

# create symbolic link
sudo ln -s /home/chiao/.composer/vendor/bin/drush /usr/local/bin
or
sudo ln -s /home/chiao/.config/composer/vendor/bin/drush /usr/local/bin

#cd ~/.config/composer
# make sure php-curl installed
sudo apt-get install php-curl -y