Fix to allow select type multiple using the bootstrap theme in Firefox/IE

The bootstrap theme uses a button for the ui-select-match-item element (the pill at the top after selecting one of the options). This causes the click event on the close 'x' to be swallowed in Firefox and IE.

Here's the multiple select demo with the original bootstrap theme. Open in firefox, click on the 'x' next to blue or red, observe that the pill does not go away.

http://plnkr.co/edit/he6uWGFARZsw66K2ERr5?p=preview

Here's the multiple select demo with my suggested updates.

http://plnkr.co/edit/3duaRkJZkJle9MH0Z3XV?p=preview