Fix mixin media-breakpoint-only

Prevent mixin media-breakpoint-only from breaking when supplying the
first or last entry in $grid-breakpoints.

scss/mixins/_breakpoints.scss

@@ -93,7 +93,13 @@
   $min: breakpoint-min($name, $breakpoints);
   $max: breakpoint-max($name, $breakpoints);
 
-  @media (min-width: $min) and (max-width: $max) {
-    @content;
+  @if $min != null and $max != null {
+    @media (min-width: $min) and (max-width: $max) {
+      @content;
+    }
+  } @else if $max == null {
+    @include media-breakpoint-up($name)
+  } @else if $min == null {
+    @include media-breakpoint-down($name)
   }
 }