Update jquery.flexslider.js to fix unresponsive arrows on touchscreens

[nickcernis]

As reported at https://wordpress.org/support/topic/arrows-not-working-2/, arrows may not work on touch devices.

Changing slider.eventType to always use click and never touchstart solves the issue: https://github.com/copyblogger/genesis-responsive-slider/blob/develop/js/jquery.flexslider.js#L28

The latest version of flexslider listens to multiple event types, so I believe that updating flexslider would solve this:

https://github.com/woocommerce/FlexSlider/blob/master/jquery.flexslider.js#L22