Update columns.sass (#1356)

SASS 3.5+ variable parsing compatibility allows only #{}

sass/grid/columns.sass

@@ -473,4 +473,4 @@ $column-gap: 0.75rem !default
     padding-right: var(--columnGap)
   @for $i from 0 through 8
     &.is-#{$i}
-      --columnGap: $i * 0.25rem
+      --columnGap: #{$i * 0.25rem}