Update Tablesorter to 2.18.3 from  http://mottie.github.io/tablesorter/

This fixes a problem with a new use of tablesorter I'm working on where the table arrows would show, but not be interactive.  One symptom was the error in the JS console: `cannot read property '0' of undefined`  In my case, the table had data, and even a Window.setTimeout() hack didn't fix it.

It was suggested to update to this fork of tablesorter in the answer to:

http://stackoverflow.com/questions/8878945/jquery-tablesorter-plugin-error-on-init-cannot-read-property-0-of-undefined