Toggle button in collapsed Navbar works again.

As `isOpen` and `isCollapsable` was changed (to `expanded` and `collapsable`) in `CollapsableMixin` the new props needs to be used to get the collapsed `Navbar` working again.

I get some graphical glitches on **Chrome37/Win7** though, that I'm not smart enough to fix;
- Expanded state shows a scroll bar with `1px` scroll space.
- Clicking fast on the toggle button locks it in place, any additional clicks won't expand/collapse the `Navbar`.

No errors on **IE11/Win7**

Best way to test is to go to the [docs page](http://react-bootstrap.github.io/) and contract the window till the `Navbar` collapses.