Update plugin.php

Getting rid of: 
Notice: register_uninstall_hook was called incorrectly. Only a static class method or function can be used in an uninstall hook. Please see Debugging in WordPress for more information. (This message was added in version 3.1.) in wp-includes\functions.php on line 2959

and subsequent: 
Warning: Cannot modify header information - headers already sent by (output started at wp-includes\functions.php:2959) in wp-includes\option.php on line 568
Warning: Cannot modify header information - headers already sent by (output started at wp-includes\functions.php:2959) in wp-includes\option.php on line 569
Merge pull request #1 from kalligator/master

Update plugin.php