Update .travis.yml for arm64 support, etc.

Update .travis.yml for arm64 support, etc.

- Add arm64 support for Linux thanks to Travis CI’s new feature, see
  https://blog.travis-ci.com/2019-10-07-multi-cpu-architecture-support
- Force GO111MODULE=on
- Cache directories
- Switch to bionic (Ubuntu 18.04) environment
- Use "master" instead of "tip" to match the latest Travis CI docs
- Allow "mage -v check" to fail on non-AMD64 architectures
  because "go test -race ./..." currently fails on arm64 and ppc64le.