Migrate to go modules and github actions

[rafibarash]

This PR:

• brings docker-credential-gcr into the future by migrating away from vendors and Travis CI towards go modules and github actions CI.
• removes the obsolete Makefile, now that we can build and test the executable with go commands (i.e. go build and go test ./...

These changes will make it easier to maintain this library in the future. Additionally, we can automate releases with GoReleaser after we've migrated to go modules.

And contrary to all the commit messages about things being removed, this PR actually improves test coverage on Windows by testing more files by removing the unit tag and Mac by testing race conditions and what were previously the travis_tests. Linux (Ubuntu) test coverage remains the same.

[rafibarash]

Binary works locally (shown below) and github action for the last commit (hash 648abe8) passed on my fork.