* chore: replace gometalinter with golangci
gometalinter is deprecated and replaced by golangci.
* chore: update go dependencies
* chore: fix lint errcheck reports
golangci-lint reported multiple errcheck issues, e.g.
```
Error return value of `p.Update` is not checked (errcheck)
```
* chore: GitHub actions replaces CircleCI
* ci: Use docker multi-stage builds
* refactor: Migrate from dep to go mod
* ci: Add Makefile to simplify builds
* ci: Upgrade to golang 1.11
* refactor: Run goimports for formatting