Update drupal core version to 8.3 (drupal-composer#13)

Author: jonathangreen

composer.json

@@ -20,7 +20,7 @@
         "composer/installers": "^1.2",
         "drupal-composer/drupal-scaffold": "^2.2",
         "cweagans/composer-patches": "~1.0",
-        "drupal/core": "8.2.*",
+        "drupal/core": "8.3.*",
         "drush/drush": "~8.0",
         "drupal/console": "~1.0",
         "drupal/devel": "^1.0@beta",