Using Go modules allows to pin versions in go.mod, so dependency management tools can track those.
Right now make targets will install latest version and travis.yml downloads binaries from github, moving to modules simplifies all of this.
tools.go is needed so that go build doesn't strip those dependencies from go.mod
Getting a diff in CI (https://travis-ci.com/prymitive/karma/jobs/167035075) as some deps are removed from go.mod
Re-create both files (go.mod & go.sum) to ensure we don't have any orphaned indirect deps there