style(tabs): add z-index to ink bar

Author: ajoslin

src/components/tabs/_tabs.scss

@@ -88,6 +88,7 @@ md-tabs[center] .md-header:not(.md-paginating) .md-header-items {
 }
 
 md-tabs-ink-bar {
+  z-index: 1;
   display: none;
   position: absolute;
   left: 0;

src/components/tabs/js/inkBarDirective.js

@@ -33,7 +33,7 @@ function MdTabInkDirective($mdConstant, $window, $$rAF, $timeout) {
       var selected = tabsCtrl.selected();
 
       var hideInkBar = !selected || tabsCtrl.count() < 2 ||
-        (scope.pagination && scope.pagination.itemsPerPage === 1);
+        (scope.pagination || {}).itemsPerPage === 1;
       element.css('display', hideInkBar ? 'none' : 'block');
 
       if (!hideInkBar) {