fix js error affecting other scripts - register jquery-core instead of jquery

on 3.9.2 (not sure what other versions), 'jquery' is registered as array('jquery-core', 'jquery-migrate')
see: https://core.trac.wordpress.org/browser/tags/3.9.2/src/wp-includes/script-loader.php#L161

The result of registering as 'jquery' was that the jquery script was inserted, then a 3rd-party script, then jquery-migrate. The 3rd party script required "$.browser". As this was not rendered until later, a JS error resulted.

The problem was actually fixed by commenting out lines 693-697, but the pull request below should be better for WP version 3.9.2. Not sure about earlier versions.