Specify a max-width for the medium grid

This prevents conflict when using col-md-* and col-lg-* classes together on the same element.

Example: 

    <div class="col-lg-2 col-md-3 col-md-pull-4">
        ...
    </div>

This causes the `col-md-pull-4` class to pull the element even when the browser window is larger than `@screen-lg-min`. This problem is fixed with this pull request.