use @screen-small var in grid.less

Per comment by @nextgenthemes on/in cd06c5ba

less/grid.less

@@ -31,7 +31,7 @@
 .generate-grid-columns(@grid-columns);
 
 // Then generate the larger grid classes via media query
-@media screen and (min-width: 768px) {
+@media screen and (min-width: @screen-small) {
   .generate-large-grid-columns(@grid-columns);
 }