hubot-jira-comment
Slack's Jira integration does not support for comments, yet.
But, Hubot can do that for you :)
Example
Jira Settings
Add an WebHook on Setting > WebHooks menu.
Hook URL is http://your-hubot-listener/hubot/chat-jira-comment/:room
Make sure the Issue Update
Events can be post.
Hubot Settings
Add this script to your hubot-slack.