Update version constant in composer.json as well.

composer.json

@@ -8,7 +8,7 @@
         "wikimedia/composer-merge-plugin": "~1.3"
     },
     "replace": {
-        "drupal/core": "~8.2"
+        "drupal/core": "~8.3"
     },
     "minimum-stability": "dev",
     "prefer-stable": true,