Move group div to allow :only-child rendering

By default, the bootstrap_mixin adds a div group in the nav div.list-group. When there are no children, this makes the `div.list-group` have 2 elements -- the `a` link that is the group and an empty `div`.  The empty div then means that `:first-child` is used by css to render the element, when in fact it should use `:only-child`.  By moving the rendering of the div inside the `each` statement, it is only rendered when there are children which allows bootstrap to correctly round all the corners with CSS.