depth must be an integer, try with this:
<?php wp_nav_menu( array(
'theme_location' => 'main_menu',
'container_id' => 'navbar-collapse-1',
'container_class'=> 'navbar-collapse collapse navbar-right',
'menu_class' => 'nav navbar-nav',
'fallback_cb' => false,
'items_wrap' => '%3$s',
'container' => false,
'depth' => 6,
'walker' => new Bunch_Bootstrap_walker()
) ); ?>
from codex:
depth (int) How many levels of the hierarchy are to be included. 0 means all. Default 0.