Migrate from Travis CI to Github actions

[ahmagdy]

Migrate build validation steps from Travis CI to Github actions.

I used two separate jobs for Linux and macOS instead of using matrix strategy as there is no docker by default in macOS. Therefore I can't run Postgres service, and I can't add a condition to exclude this service. Conditions are only supported for steps and not for services!

It's very annoying that I had to duplicate go build and test steps because there are no composite actions in Github yet. It's being tracked in https://github.com/actions/runner/issues/646.

The workflow has been tested in https://github.com/ahmagdy/go-askme/actions/runs/627285467.