Fix for slackdiff.

[Punicaa]

Pre-Request Checklist

• Passes rubocop code analysis (try rubocop --auto-correct)
• Tests added or adapted (try rake test)
• Changes are reflected in the documentation
• User-visible changes appended to CHANGELOG.md

Description

• Updated slackdiff.rb to use slack_ruby_client instead of slack-api.
• Updated default timeout from 20 to 60.

Fixes users experiencing the following error:

ERROR -- : Hook slack (#SlackDiff:0x00005572c9621e20) failed (#<NameError: uninitialized constant Slack::Client>) for event :post_store #2697

[robertcheramy]

I miss the update of the default timeout in the diff. Can you check that?
Can you also merge master into your branch (or rebase it, as you which), and modify Changelog so that the changes are under the unreleased Version?