update Hero Panel preview to use new master.* and not old versions in _files

[CharlesChapman]

Make necessary updates to /_cascade/formats/Homepage Panel Preview so it uses the new master.js and master.css from github instead of the old styles_2013.css etc. This is only used for Preview inside Cascade for individual Hero Panels but should be consistent with live homepage so we aren't using those old js and css files anywhere on site and can retire them