Use Github Actions for CI

[balexand]

The current .travis.yml is using the old and unsupported Elixir version of 1.4 and an old version of OTP too. This pull-request switches to Github Actions and builds on a matrix of all supported Elixir versions. If https://github.com/c0b/docker-elixir/issues/122 is resolved then the matrix could also include OTP versions.

If this is merged then you'll probably want to disable Travis CI on this repo. Thanks ❤️

[balexand]

You can see the logs for this workflow running in my fork here: https://github.com/balexand/shopify/commit/88ae058c52eb5d09c361f54bfe8c432d54693947/checks?check_suite_id=338213121