test(sortable.spec): add simple $apply test to increase code coverage.

Only [line 88](https://github.com/angular-ui/ui-sortable/blob/master/src/sortable.js#L88) is not tested with jquery-ui 1.10, since it's a workaround to support jquery-ui 1.9.
When using jquery-ui 1.9, code coverage is 100%.