Update subtask-dragger.js

jQuery drag and drop functionality for Jira subtasks
Inspired on version of anorakgirl (https://github.com/anorakgirl/subtask-dragger) with modifications by czerwiu:
- reordering subtasks without reloading the page
- visual DnD handle
- fixed issue with moving subtask to the last position
- works with Jira 5.x and 6.x