Migrate from jslack into java-slack-sdk

[gonchik]

Hi!

Any plans to migrate into java-slack-sdk ?
https://github.com/slackapi/java-slack-sdk

It would be nice to sleep better with compatibility of slack api changes

Cheers,
GT

[mvlasovatl]

Hey @gonchik !
I took a look at the offered library and found that its API is similar to the API of the client currently used but it is not backward compatible with it. It means that to migrate to the new library we'd have to refactor a lot of code. It would take long and require a lot of efforts, but wouldn't bring any sufficient benefits. Currently used jslack client is old, but it is still compatible with Slack REST API, all plugin features work fine. We may consider taking the migration path if Slack plugin stops working due to Slack REST API changes though.

If you want to help, please feel free to fork the repo and submit a PR with the new client integrated.

[mvlasovatl]

After #228 this change looks more appealing.