Fix for #1518. Update to select2.js to trigger properly named event.

By the documentation the event that should be fired is "select2-removed" however the event that actually gets fired is called "removed"
Update to unselect method to support "select2-removing" triggers.

The goal here is send off a triggered event to a subscriber that an
element is going to be removed. It hasn't removed yet but would be
after the event fires. I have also added logic to determine if a
subscriber has prevented default. This will prove useful in the future
if a subscriber needs to extend the functionality for other usages.
Merge pull request #1774 from avelis/master

Fix for #1518. Update to select2.js to trigger properly named event.