How can I get "divider" displayed for "nav-pills" and "nav-stacked"

<li class="nav-item"><hr style="width:90%;"></li>

Because of the deprecation. Change according to your liking.

The divider class has changed to nav-divider in Bootstrap 3..

 <li class="nav-divider"></li>

So you can use this without any custom CSS. http://bootply.com/101379

UPDATE

.nav-divider has been deprecated in v3.0.1

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