Consolidate on GitHub actions

[swalkinshaw]

• I've read the guidelines for Contributing to Roots Projects
• This request isn't a duplicate of an existing request
• This is not a personal support request that should be posted on the Roots Discourse community

Summary

We should consolidate the build process on GitHub actions instead of a confusing mix of AWS Lambda + Travis CI

Motivation

Simplicity so everything is in one place

[swalkinshaw]

Done as of #28

If we need to, we can manually trigger workflow runs or re-enable the old method (AWS Lambda).