Fix incorrectly named navbar-collapse class

I'm thinking `nav-collapse` might have been the class in Bootstrap 2? [Bootstrap 3 now uses `navbar-collapse`](https://github.com/seyhunak/twitter-bootstrap-rails/blob/bdd3063d5db19057b843cd4b8c0c6f71697b385d/vendor/assets/stylesheets/twitter-bootstrap-static/bootstrap.css.erb#L3658).

This fixes the bug where the navbar isn't visible when collapsed.