fix(carousel): use proper sort comparison

Fixes #5638
Closes #5702

Author: charlierudolph

src/carousel/carousel.js

@@ -15,7 +15,7 @@ angular.module('ui.bootstrap.carousel', [])
       element: element
     });
     slides.sort(function(a, b) {
-      return +a.slide.index > +b.slide.index;
+      return +a.slide.index - +b.slide.index;
     });
     //if this is the first slide or the slide is set to active, select it
     if (slide.index === $scope.active || slides.length === 1 && !angular.isNumber($scope.active)) {