Fix notice: WritingHelper::$supported_post_types

```
[09-Dec-2013 18:15:35 UTC] PHP Notice:  Undefined property: WritingHelper::$supported_post_types in /srv/www/vocativ.dev/content/plugins/writing-helper/writing-helper.php on line 53
```

Variable was removed in 6596d9ca0e7a3f8a158849d701e3b1211a3626fe