fix(sortable): race condition when removing from DOM

There is a race condition when an ui-sortable element is removed from the DOM, because the angular wrapper uses a `$timeout` to call the `refresh` on the jquery plugin, which is not possible after the element has been removed from DOM.
This patch just ensure the element is still "pluginified" before making the final call.

Sample thrown error :
```js
Error: cannot call methods on sortable prior to initialization; attempted to call method 'refresh'
    at Function.jQuery.extend.error (http://127.0.0.1:9393/bower_components/jquery/dist/jquery.js:248:9)
    at HTMLUListElement.<anonymous> (http://127.0.0.1:9393/bower_components/jquery-ui/ui/jquery-ui.js:487:15)
    at Function.jQuery.extend.each (http://127.0.0.1:9393/bower_components/jquery/dist/jquery.js:381:23)
    at jQuery.fn.jQuery.each (http://127.0.0.1:9393/bower_components/jquery/dist/jquery.js:137:17)
    at $.widget.bridge.$.fn.(anonymous function) [as sortable] (http://127.0.0.1:9393/bower_components/jquery-ui/ui/jquery-ui.js:483:9)
    at http://127.0.0.1:9393/bower_components/angular-ui-sortable/sortable.js:40:23
    at http://127.0.0.1:9393/bower_components/angular/angular.js:13677:28
    at completeOutstandingRequest (http://127.0.0.1:9393/bower_components/angular/angular.js:4193:10)
    at http://127.0.0.1:9393/bower_components/angular/angular.js:4494:7 
```