Fixed bug in require path for GELF (Graylog2)

require_once 'lib/Graylog2-gelf-php/...' is not correct because the Magento root is not always in the include_path.
The directory "lib" is in the include_path, so removing "lib/" fixes this problem.
The problem occurred when you ran a Magento script which is not located in the Magento root, for example a shell script.