use base api endpoint to verify token is valid

src/Composer/Util/GitHub.php

@@ -104,7 +104,7 @@ class GitHub
         try {
             $apiUrl = ('github.com' === $originUrl) ? 'api.github.com' : $originUrl . '/api/v3';
 
-            $this->remoteFilesystem->getContents($originUrl, 'https://'. $apiUrl . '/rate_limit', false, array(
+            $this->remoteFilesystem->getContents($originUrl, 'https://'. $apiUrl, false, array(
                 'retry-auth-failure' => false,
             ));
         } catch (TransportException $e) {