fix undefined "shipped"

Fixes:
```
[Thu Oct 17 11:33:18 2013] [error] [client ::1] PHP Notice:  Undefined index: shipped in /srv/http/core/lib/private/app.php on line 775, referer: http://localhost/core/index.php/setting
s/apps?installed
[Thu Oct 17 11:33:18 2013] [error] [client ::1] PHP Stack trace:, referer: http://localhost/core/index.php/settings/apps?installed
[Thu Oct 17 11:33:18 2013] [error] [client ::1] PHP   1. {main}() /srv/http/core/index.php:0, referer: http://localhost/core/index.php/settings/apps?installed
[Thu Oct 17 11:33:18 2013] [error] [client ::1] PHP   2. OC::handleRequest() /srv/http/core/index.php:30, referer: http://localhost/core/index.php/settings/apps?installed
[Thu Oct 17 11:33:18 2013] [error] [client ::1] PHP   3. OC_Router->match() /srv/http/core/lib/base.php:658, referer: http://localhost/core/index.php/settings/apps?installed
[Thu Oct 17 11:33:18 2013] [error] [client ::1] PHP   4. call_user_func() /srv/http/core/lib/private/router.php:127, referer: http://localhost/core/index.php/settings/apps?installed
[Thu Oct 17 11:33:18 2013] [error] [client ::1] PHP   5. __lambda_func() /srv/http/core/lib/private/router.php:127, referer: http://localhost/core/index.php/settings/apps?installed
[Thu Oct 17 11:33:18 2013] [error] [client ::1] PHP   6. require_once() /srv/http/core/lib/private/route.php(121) : runtime-created function:1, referer: http://localhost/core/index.php/
settings/apps?installed
[Thu Oct 17 11:33:18 2013] [error] [client ::1] PHP   7. OC_App::listAllApps() /srv/http/core/settings/js/apps-custom.php:19, referer: http://localhost/core/index.php/settings/apps?inst
alled
[Thu Oct 17 11:33:18 2013] [error] [client ::1] PHP   8. usort() /srv/http/core/lib/private/app.php:756, referer: http://localhost/core/index.php/settings/apps?installed
[Thu Oct 17 11:33:18 2013] [error] [client ::1] PHP   9. OC_App::customSort() /srv/http/core/lib/private/app.php:756, referer: http://localhost/core/index.php/settings/apps?installed
```

cc @karlitschek