Replace Travis with Github Actions for CI/CD (closes #117)

[mscwilson]

Tests against a variety of Rubies using setup-ruby action, and publishes to RubyGems if a commit is tagged with the correct release version. Tests failed when using Ruby 2.0.

Uses Coveralls action for test coverage. I followed this solution to set up Simplecov for Coveralls.

[mscwilson]

As I mentioned here, the Gemfile.lock is not being committed at the moment. We would need to add some extra config for the test matrix to include it. But perhaps this could be its own Issue to consider.

[paulboocock]

Looking good! I only have two comments:

1. Add a Github Release to the publish step (or a new release step). e.g. https://github.com/snowplow-incubator/snowplow-react-native-tracker/blob/master/.github/workflows/deploy.yml#L186
2. Split the ruby.yml into two workflows, test.yml and deploy.yml - this creates a better separation, although it does create some duplication. It allows the on to be more specific for each, rather than relying on the if statements which are a little hidden. I'm not too fussed though, both approaches work fine.