Add this to the bottom of your wp-config.php file:
if(is_admin()) { add_filter('filesystem_method', create_function('$a', 'return "direct";' )); define( 'FS_CHMOD_DIR', 0751 );}
change the ownership of the Wordpress folder (that contains your wordpress installation) to "_www" user which is the owner user of apache httpd service:
sudo chown -R _www wordpress
sudo chmod -R 755 wordpress