(3.x) GitHub actions and some other improvements

[pedrofurtado]

Hello, @stevenosloan @siegy22 ! 👋

Thank you for your hard work on this gem.

I opened this PR to propose some changes and improvements, such as:

• Replacement of Travis CI to fully integrated CI/CD with Github Actions. It includes tests, lints, and deploy to Rubygems. The last one (deploy to Rubygems), once configured, can be made by "Actions" tab, dispatching the workflow manually or be triggered by a release/tag creation on repo. The two option are possible;
  • Example of CI output: https://github.com/pedrofurtado/slack-notifier/actions/runs/1331169528
• Configuration to rubocop run in all files except specs and gemspec, and fixes of offenses;
• Dockerfile for other devs pull repo, make changes and test it in PC easily;
• Renames of some special files (CHANGELOG, README for example) and some text changes;
• Exclusion of feature for multiple-channel message sending, because Slack does not provide this anymore;
• README improvements

Let me know what you think about 🍻 🤝

PS: Some secrets are required to be configured, for Rubygems deploy. I can explain for you the steps to be done 👍

PS 2: The gem internal structure was changed, because of this I've changed version to 3.x 🔍

PS 3: Dont worry the amount of files changed 😄 , the gem keeps stable and with test suite running well in Rubies from 2.0 until 3.0, JRuby and TruffleRuby 🎉

[pedrofurtado]

Your eyes/review on it will be awesome 🕺 🏆 @stevenosloan @siegy22

[pedrofurtado]

👀

[siegy22]

Thank you very much for working on this, I need @stevenosloan to respond first if he's willing to give maintenance control to me. 😄

[pedrofurtado]

@siegy22 🎉 🤝

[stevenosloan]

@siegy22 sorry about that, you should have an invite to be a maintainer -- I *thought* I'd done that when I split this off to an org instead of my personal, but that appears not to have been the case 😬

[pedrofurtado]

Any news about this PR? 👀 😄

[siegy22]

@stevenosloan Can you re-send the invitation, I tried accepting it, but it was already expired 😞