Migrate the mlavalon Repo to Use Github Actions for CI

[jasoncorum]

Since Travis CI moved to a pay-model, and we've exhausted our free credits, we need a new CI solution. After some preliminary investigating and implementing on AAPB, Github Actions seems like our best bet.

The AAPB2 repo has a PR #2332 which uses Github Actions for CI. It sets up the machine, runs RSpec, and Rubocop. This can be used as a starting place for our other repositories.

DONE WHEN the mlavalon repo uses Github Actions for CI

• mlavalon - https://github.com/WGBH-MLA/mlavalon