Fix sass map navbar-toggleable

Follow up to 6e8ce0c7dc. Uses the breakpoint-next mixin to snag the next breakpoint in the media query for proper styling.

scss/_navbar.scss

@@ -133,9 +133,10 @@
 // scss-lint:disable ImportantRule
 .navbar-toggleable {
   @each $breakpoint in map-keys($grid-breakpoints) {
+    $next: breakpoint-next($breakpoint, $grid-breakpoints);
     &-#{$breakpoint} {
       @include clearfix;
-      @include media-breakpoint-up($breakpoint) {
+      @include media-breakpoint-up($next) {
         display: block !important;
 
         .navbar-brand {