Update extensions.php

Fix of "NOTICE: PHP message: PHP Notice:  Array to string conversion in /.../admin/extensions.php on line 919" in PHP 5.4

http://punbb.ru/topic5405-notice-php-message-php-notice-array-to-string-conversion.html
Merge pull request #95 from Allineer/patch-1

Fix of NOTICE: PHP message: PHP Notice: Array to string conversion