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 Bump RuboCop to v1.18.x via rubocop-jekyll #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 Bump RuboCop to v1.18.x via rubocop-jekyll #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