Fix for latest jquery version.

[mherbold]

Read up on prop vs. attr. This little fix was needed to get "add all" working again for our version of jQuery (1.11). I am sure there are other prop vs. attr issues with this plugin, I did not check or test the others.

[ejegg]

I think the second argument to 'prop' needs to be boolean. Also, there are a bunch of other places attr is used with selected. Here's an example of a local fix to multiselect from wikimedia: https://gerrit.wikimedia.org/r/#/c/406598/1/resources/vendor/jquery.ui.multiselect/ui.multiselect.js