Update sortablelist.js

Updated line 89 from 

formData = formData.replace('task', '');
to
formData = formData.replace('task=', '=');

because the former version causes issues if the extension developer has any form fields that contain the string "task" in it's name. So whenever we have a form field like "task_id" or something similar, the sortable functionality does not work, because the task field is not being removed from the POST data.

This change should fix this.