Prepare for upcoming format break

composer.lock

@@ -1,5 +1,5 @@
 {
-    "hash": "4c4d0c5fe9463c0bc507921d46045821",
+    "hash": "25c80534204714cf48b2e67611a370c5",
     "packages": [
         {
             "package": "justinrainbow/json-schema",

src/Composer/Repository/ComposerRepository.php

@@ -51,6 +51,11 @@ class ComposerRepository extends ArrayRepository
             throw new \UnexpectedValueException('Could not parse package list from the '.$this->url.' repository');
         }
 
+        if (isset($packages['includes'])) {
+            $this->io->write('<error>Your version of composer is too old, please run `php composer.phar self-update` to update it.</error>');
+            exit(1);
+        }
+
         $loader = new ArrayLoader();
         foreach ($packages as $data) {
             foreach ($data['versions'] as $rev) {