Migrate Travis CI from Travis-CI.org to Travis-CI.com

Travis-CI.org will be shut down on December 31, 2020. Full details are in [this newsletter from Travis](https://mailchi.mp/3d439eeb1098/travis-ciorg-is-moving-to-travis-cicom?e=%5BUNIQID%5D).

Now is the time to migrate to Travis-CI.com.

Instructions for migrating to Travis-CI.com are available in the [Travis documentation on repository migration](https://docs.travis-ci.com/user/migrate/open-source-repository-migration).

In addition to migrating the repository to Travis-CI.com, please remember to also update the Travis status badges in the README.md files, make sure that all of the README.md files are using the Travis-CI.com badges. More information on Travis status badges is available in the [Travis documentation page on status images](https://docs.travis-ci.com/user/status-images/).

Reference: https://discourse.julialang.org/t/reminder-travis-ci-org-shuts-down-on-december-31-2020/47234

Read the Travis repository migration and status-image documentation linked in the issue, then inspect the repository's Travis CI setup and all README.md files. Migrate the repository to Travis-CI.com and update every Travis status badge to use the .com service; verify the affected files and CI status afterward.

索引モデルが issue の本文から書いたものです。