Update CI & release process

[dylanjha]

1. Rename master branch to main
2. Update release process so that it happens through GH actions
3. A release should get triggered by creating a new release in github muxinc/mux-node-sdk is a good example
4. Update CI to happen through github actions instead of travis, expand test matrix so that it tests various versions of elixir and erlang (right now it only runs tests on elixir: '1.6 and otp_release: '20.0')
5. Move release instructions out of README.md and into a README_INTERNAL.md