Applied fix to destroy error with JQuery 1.9.

Closes issue: https://github.com/mleibman/SlickGrid/issues/484

Prevents and error generated when destroy is called multiple times on the $headers variable due to the .sortable property still being true after destroy has previously been called in JQuery 1.9.

    Error: cannot call methods on sortable prior to initialization; attempted to call method 'destroy'