Variation table class orderby to 'menu_order title'

Changed orderby to 'menu_order title' because post_title isn't a proper orderby clause for WP_Query
Update wpsc-admin/includes/product-variation-list-table.class.php
Merge pull request #74 from joshlevinson/patch-1

Variation table class orderby to 'menu_order title'.  Props @joshlevinson