Migrate CI from Travis CI to GitHub Actions

[karlkurzer]

Migrate CI from Travis CI to GitHub Actions

• Removed .travis.yml since Travis CI is no longer offering a free tier
• Added .github/workflows/ci.yml using ros-tooling/setup-ros and
  ros-tooling/action-ros-ci to run the test suite on Ubuntu 20.04 and
  ROS Noetic.
• Replaced the Travis CI badge with the GitHub Actions badge in the README.md

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PR created automatically by Jules for task 13366481632090692117 started by @karlkurzer

[google-labs-jules]

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