Work around a bug in composer that selects a feature branch of Robo.

We do not directly depend on Robo, but adding this line ensures that if we
update dependencies using the --prefer-lowest flag then Composer will
not accidentally select the `states` feature branch. It is likely that
this bug is caused by the presence of the alias "dev-state": "1.x-dev" in
the composer manifest of Robo.

Author: pfrenssen

composer.json

@@ -10,6 +10,7 @@
     },
     "require-dev": {
         "composer/installers": "~1.2",
+        "consolidation/robo": "~1.3",
         "drush/drush": "~9.4",
         "drupal-composer/drupal-scaffold": "~2.2",
         "instaclick/php-webdriver": "^1.4.5",