Trying to introduce travis-ci.com with GitHub App

[KengoTODA]

I'm now trying to introduce https://travis-ci.com/spotbugs/spotbugs instead of travis-ci.org.

reference:

• https://blog.travis-ci.com/2018-05-07-announcing-support-for-github-checks-api-on-travis-ci-com
• https://blog.travis-ci.com/2018-05-02-open-source-projects-on-travis-ci-com-with-github-apps
• https://docs.travis-ci.com/user/open-source-on-travis-ci-com

[KengoTODA]

It seem that migration will be handled by Travis CI themselves.

https://docs.travis-ci.com/user/open-source-on-travis-ci-com/#Existing-Open-Source-Repositories-on-travis-ci.org