Remove/migrate all remaining Travis stuff

[slarse]

As we're unlikely to ever return to Travis CI, references to Travis are confusing, e.g. the directory containing the CI scripts being called chore/travis and the .travis.yml config file being present. We should remove anything that is no longer necessary, and rename everything else appropriately.

In the process of removing the Travis CI traces, we should also ensure that we've migrated all functionality that we want over to GitHub Actions.

Known other problems:

• The "external CI" scripts are not being pulled as they depend on Travis environment variables:

  spoon/chore/travis/ci-extra.sh

  Lines 101 to 105 in 4880da0

  	if [[ "$TRAVIS_REPO_SLUG" == "INRIA/spoon" ]] && [[ "$TRAVIS_PULL_REQUEST" != "false" ]]
  	then
  	echo "downloading extra CI PR script from SpoonLabs/spoon-ci-external"
  	curl https://raw.githubusercontent.com/SpoonLabs/spoon-ci-external/master/spoon-pull-request.sh | bash
  	fi

[KishkinJ10]

hello sir , i am a beginner and i am participating in the GSoC 2021 with Project Spoon , sir can you please brief me what are we going to do in this project ?? and how can i write timeline section in my proposal?

[slarse]

@KishkinJ10 sorry, I missed this comment. Hope you found your way to the GSoC issue #3828.

Closing this as #3834 removed the final traces of Travis.