Orbit: `min-height` for slides_container (instead of `height`)

Using `min-height` (with `position:relative`) allows the slide container
to get expanded, in case it contains elements of variable height (e.g.
accordions).
Merge branch 'orbit-minheight' of github.com:blueyed/foundation into blueyed-orbit-minheight
Merge branch 'master' of github.com:zurb/foundation