Update to Drush 9 for compatibility with Drupal core 8.4.x
Parallel issue: drutopia_template#3 (closed)
Core now uses a later version of Synfony, so we need to use Drush 9.
Parallel issue: drutopia_template#3 (closed)
Core now uses a later version of Synfony, so we need to use Drush 9.