wp_nav_menu in 3.7 add name class nav navbar-nav

Edit

In this case, it appears that the problem is that you’re not actually outputting a custom nav menu at all, but rather the menu fallback: wp_page_menu(). Notice that the classes for the list items are page-item rather than menu-item. That’s a sure giveaway that the output is being generated by wp_page_menu() rather than wp_nav_menu().

Solution: Ensure that you have created a custom nav menu, and assigned it to the menu_principal Theme Location.

Original Answer

Long answer, short: you’re using the correct method to modify menu_class, and in the process, likely stepping on changes that were made to wp_nav_menu() between WordPress 3.6 and 3.7.

Instead of messing with items_wrap, you need to be manipulating menu_class, which is the correct parameter for defining the class applied to the menu element output by wp_nav_menu():

$menu_class

(string) (optional) The class that is applied to the ul
element which encloses the menu items. Multiple classes can be
separated with spaces. Formerly known as $wrap_class.

Default: menu

Try this:

<?php wp_nav_menu( array(
    'container' => false,
    'menu_class' => 'nav navbar-nav',
    'theme_location' => 'menu_principal'
) ); ?>