Don't use Collection.models.indexOf

Arrays in IE < IE9 don't have an indexOf method, so calls to
``Collection.models.indexOf`` cause an error in IE8. Luckily there's a
``Collection.indexOf`` method that uses Underscore and offers a fallback
implementation for browsers that don't implement indexOf.

Closes #766
Merge branch 'hotfix-766-viewer_broken_in_ie8' into develop

Fix for #766