remove block-grid bullets in IE

I know Zurb dropped support for IE8, but maybe you'll accept this small change that improves IE8 users' life a little bit and should not have side effects.

Basically, in IE9+ and other modern browsers `display: inline` on a `<li>` will not include the bullet point (because they are not `display: list-item` anymore, so it makes sense).

But IE8 does display the bullets, because it's IE8.

This PR just sets `list-style: none;` for the `li`'s in `.block-grid`'s