CI: travis-ci -> github actions

[leonnicolas]

It looks like travis is not running CI for this project anymore. This PR moves the CI to github actions.

• use t.Helper() to not get confused with where the error happened in a test.
• require go 1.11 (go mod is not available in earlier versions)

Note: to run test for later go version > 1.13, we would have to update an expected error in embedmd_test.go.