Shouldn't assume result of `get_preferred_from_update_core` is an object. fixes #61

Notice: Trying to get property of non-object in /srv/www/wp-content/plugins/developer/developer.php on line 847

Throws an error when get_preferred_from_update_core returns false
Merge pull request #62 from jtsternberg/master

Shouldn't assume result is an object. fixes #61

props to @jtsternberg
Remove mp6

Now part of core as of WordPress 3.8.