Migrate Travis CI by May 31, 2021

[junaruga]

Hi. I see this repository still uses travis-ci.org.

Note Travis-ci.org shut down will occur on May 31, 2021.
https://blog.travis-ci.com/2021-05-07-orgshutdown

The arch: arm64, arch: ppc64le, arch: s390x free unlimited builds for all the open source repositories was announced yesterday. If you migrate this repository to travis-ci.com, we can still use these CPU architectures for free even after the May 31, 2021.
https://docs.travis-ci.com/user/billing-overview/#partner-queue-solution

A migration path I can recommend is

1. Migrate AMD64 cases to GitHub Actions.
2. Remove AMD64 cases on Travis, because it consumes credits. Then migrate travis-ci.org to tarvis-ci.com. You can migrate it on the setting page.
3. Use non-AMD64 cases on travis-ci.com.

Here is an example of GitHub Actions.
https://github.com/brianmario/mysql2/blob/master/.github/workflows/build.yml
https://github.com/ruby/ruby/tree/master/.github/workflows

Here is an example of travis-ci.com credits page. You can check a history of builds with consumed credits.
https://travis-ci.community/t/free-oss-credits-usage-calculation/10832/4

Here is my repository to test the CPU architectures on travis-ci.com. I confirmed it is free.
https://github.com/junaruga/ci-multi-arch-native-test