Make vendor writable

Closes #3501187

.tugboat/config.yml

@@ -30,6 +30,7 @@ services:
         - chmod 2775 $TUGBOAT_ROOT/project
         - chgrp -R www-data $TUGBOAT_ROOT/project
         - chmod g+w $TUGBOAT_ROOT/project/web
+        - chmod g+w $TUGBOAT_ROOT/project/vendor
         # Install Drupal using Drush.
         - |
           set -e