Adds not function for Libsass compatibility

- Closes #254

app/assets/stylesheets/functions/_new-breakpoint.scss

@@ -41,7 +41,7 @@
     $query: append($query, $total-columns);
   }
 
-  @if not belongs-to($query, $visual-grid-breakpoints) {
+  @if is-not(belongs-to($query, $visual-grid-breakpoints)) {
     $visual-grid-breakpoints: append($visual-grid-breakpoints, $query, comma) !global;
   }
 

app/assets/stylesheets/functions/_private.scss

@@ -1,15 +1,21 @@
+// Not function for Libsass compatibility
+// https://github.com/sass/libsass/issues/368
+@function is-not($value) {
+  @return if($value, false, true);
+}
+
 // Checks if a number is even
 @function is-even($int) {
-  @return $int % 2 == 0
+  @return $int % 2 == 0;
 }
 
 // Checks if an element belongs to a list or not
 @function belongs-to($tested-item, $list) {
-  @return not not-belongs-to($tested-item, $list);
+  @return is-not(not-belongs-to($tested-item, $list));
 }
 
 @function not-belongs-to($tested-item, $list) {
-  @return not index($list, $tested-item);
+  @return is-not(index($list, $tested-item));
 }
 
 // Contains display value

app/assets/stylesheets/grid/_media.scss

@@ -68,7 +68,7 @@
     $default-grid-columns: $grid-columns;
     $grid-columns: $total-columns !global;
 
-    @if not is-even(length($query)) {
+    @if is-not(is-even(length($query))) {
       $grid-columns: nth($query, $loop-to) !global;
       $loop-to: $loop-to - 1;
     }