Add display block to our popover arrows

Author: aaronshekey

lib/css/components/_stacks-popovers.less

@@ -112,6 +112,7 @@
 .s-popover--arrow:before,
 .s-popover--arrow:after {
     position: absolute;
+    display: block;
     width: @su12;
     height: @su12;
     z-index: -1;