Switch from Travis CI to GitHub Actions

[emmahsax]

Switch from Travis CI to GitHub Actions. This will join a slow shift in Jekyll's projects from Travis CI to GitHub Actions.

I also prefer to use GitHub Actions because Travis CI (at least on travis-ci.com) is now charging money for users to use more than 10K credits.

[emmahsax]

@parkr or @DirtyF or @ashmaroli

I've created this pull request to help move this repository from Travis CI to GitHub Actions. Can you please take a look when you have a chance?

I also don't know how to make my pull request trigger GitHub Actions workflows... that may be a setting in the repository, but they all passed on my fork's pull request except the Rubocop test.

[ashmaroli]

@emmahsax Thank you for initiating the migration. The reason your workflow did not trigger a run on this repo is probably because of:

• There wasn't an existing workflow file on the default branch here.
• Your workflow is in a fork. (Technically, not part of this repo yet).
• Your first contribution here. (Unlikely a reason, but possible).

I have now pushed a workflow file to master, to have GH Actions recognize pull requests from forks to this repo. Please rebase your branch along with the following changes:

• The if: "!contains(github.event.commits[0].message, '[ci skip]')" is no longer necessary. GH Actions have it built-in now.
• You need not configure new jobs. Keep it DRY (as much as possible) using matrix.include. Refer #291 for an example.
• Default checkout depth is fine. 1000 commits deep is ....
• Matrix entry 3.0 needs to be quoted "3.0". So quote all entries.
• Prefer double-quoted strings consistently.
• Leave out running RuboCop from the matrix for this pull request. The existing config is old. I'll update after merging your PR and CI is successfully online. (See #291)

[emmahsax]

@ashmaroli Thank you for the response! I have made the suggestions you mentioned. I suppose I need approval to run workflows on this pull request, but then they should trigger 🤞🏼 .

Thank you!

[emmahsax]

@ashmaroli All right, the tests have all run and passed. This PR is ready for another review!

[ashmaroli]

Thank you @emmahsax @jekyllbot: merge +dev