Enable display of dropdown navbar links when @grid-float-breakpoint has been adjusted.

After changing @grid-float-breakpoint to, say, @screen-md-min, the dropdown menus won't appear since the CSS which displays them is enclosed in
```css
@media (max-width: @screen-xs-max) {
```
This pull request changes the media query to
```css
@media (max-width: @grid-float-breakpoint) {
```
which fixes it.