Use GitHub Actions instead of Travis CI

.github/CODEOWNERS

.github/workflows/build-linux.yml

.github/workflows/build-macos.yml

.travis.yml

README.md

docs/source-code-overview.rst

src/CMakeLists.txt

utils/github/bundle-macos.sh

utils/github/coverage-linux.sh

utils/github/install-linux.sh

utils/github/test-linux.sh

utils/travis/after_success-osx.sh

utils/travis/before-install-linux.sh

utils/travis/before-install-osx.sh

utils/travis/install-osx.sh

utils/update_travis-mac_branch.sh

version.cmake