Fix Strict Standards Error in Upboot_Walker_Nav_Menu

Fixes error "Strict Standards: Declaration of Upbootwp_Walker_Nav_Menu::start_lvl() should be compatible with Walker_Nav_Menu::start_lvl(&$output, $depth = 0, $args = Array) in /path/to/wordpress/wp-content/themes/upBootstrap3WP/inc/general/class-Upbootwp_Walker_Nav_Menu.php on line 152", which was mentioned in Issue 5 (https://github.com/upplex/upBootstrap3WP/issues/5).