Be consistent in mobile view for product listing one by line / two by…

… line. Fixes #564

templates/catalog/_partials/productlist.tpl

@@ -2,7 +2,7 @@
  * For the full copyright and license information, please view the LICENSE
  * file that was distributed with this source code.
  *}
-{capture assign="productClasses"}{if !empty($productClass)}{$productClass}{else}col-xs-6 col-xl-4{/if}{/capture}
+{capture assign="productClasses"}{if !empty($productClass)}{$productClass}{else}col-6 col-xl-4{/if}{/capture}
 
 <div class="products{if !empty($cssClass)} {$cssClass}{else} row{/if}">
   {foreach from=$products item="product" key="position"}