Remove the use of jQuery's is() function for jQuery < 1.6 support.

The Chosen.prototype.test_active_click function has a dependency on jQuery 1.6's version of the .is() function. Prior to 1.6, jQuery's .is() function works with string selectors, but it does not work with DOM or jQuery objects passed as the parameter (see http://api.jquery.com/is/).

Code change from HitmanInWis at https://github.com/harvesthq/chosen/issues/1471