AngularJS ng-repeat rerender

I'm building a simple app with AngularJS. The app make a async AJAX call to the server and the server returns an array like this:

{
    paragraphs: [
        {content: "content one"},
        {content: "cnt two"},
        {content: "random three"},
        {content: "last one yeeaah"}
    ]
}

So I'm setting this content to the StorageService factory via my set method. Everything is fine here.

I'm using ng-repeat to render the results and JQuery UI sortable to be able to change the order of the elements. When an item is swapped my script is calling the StorageService.swap method and the element order in StorageService is updated, BUT ng-repeat isn't rerendering the change, but if I remove/add or change the content it's working. How I can force the angular to rerender the ng-repeat?

= JSFIDDLE =

http://jsfiddle.net/6Jzx4/3/

= Example =

When a swap occurs the ng-repeat should be rerendered, so the IDs are consecutive

= Code =

HTML

<div ng-controller="Test" sortable>
    <div ng-repeat="item in data().paragraphs" class="box slide_content" id="{{$index}}"> 
        {{item.content}}, ID: {{$index}}
    </div>

    <input type="button" ng-click="add()" value="Add">
</div>

JS

var App = angular.module("MyApp", []);

App.controller("Test", function($scope, StorageService) {
    StorageService.set({
        paragraphs: [
            {content: "content one"},
            {content: "cnt two"},
            {content: "random three"},
            {content: "last one yeeaah"}
        ]
    });

    $scope.data = StorageService.get;

    $scope.add = StorageService.add;
});

App.directive("sortable", function(StorageService) {
    return {
        link: function(scope, element, attrs) {
            $(element[0]).sortable({
                cancel: ".disabled",
                items: "> .slide_content:not(.disabled)",
                start: function(e, t) {
                    t.item.data("start_pos", t.item.index());
                },
                stop: function(e, t) {
                    var r = t.item.data("start_pos");
                    if (r != t.item.index()) {
                                    StorageService.sort($(this).sortable("toArray"));
                    }
                }
            });
        }
    };
});

App.factory('StorageService', function() {
    var output = {};

    return {
        set: function(data) {
            angular.copy(data, output);
            return output;
        },
        get: function() {
            return output;
        },
        add: function() {
            output.paragraphs.push({
                content: 'Content'
            });
        },
        sort: function(order) {
            var localOutput = [];

            for (var j in order) {
                var id = parseInt(order[j]);
                localOutput.push(output.paragraphs[id]);
            }

            console.log('new order', localOutput);

            output.paragraphs = localOutput;
            return output;
        }
    };
});