Add gitlab.com as default GitLab domain

src/Composer/Config.php

@@ -43,6 +43,7 @@ class Config
         'prepend-autoloader' => true,
         'github-domains' => array('github.com'),
         'github-expose-hostname' => true,
+        'gitlab-domains' => array('gitlab.com'),
         'store-auths' => 'prompt',
         // valid keys without defaults (auth config stuff):
         // github-oauth