Add CI fixes and improvements (#780)

* Update GolangCI-lint and lint config

The `algo` parameter for the `unparam` linter has been removed and we
should thus no longer specify it. Also, bumping the GolangCI-lint
version to the latest available minor release.

See: mvdan/unparam@e6a6d1c51b

* Fix and improve bintray CI script

* Further CI setup improvements

* Split-out CI steps into stand-alone scripts

ci/lint.sh

@@ -0,0 +1,17 @@
+#!/usr/bin/env bash
+set -u -e -x -o pipefail
+
+if [[ -n "${GOLANGCI_VERSION-}" ]]; then
+  # Retrieve the golangci-lint linter binary.
+  curl -sfL https://install.goreleaser.com/github.com/golangci/golangci-lint.sh | bash -s -- -b ${GOPATH}/bin ${GOLANGCI_VERSION}
+fi
+
+# Run the linter.
+golangci-lint run
+
+if [[ "${GO111MODULE-off}" == "on" ]]; then
+  # If Go modules are active then check that dependencies are correctly maintained.
+  go mod tidy
+  go mod vendor
+  git diff --exit-code --quiet || (echo "Please run 'go mod tidy' to clean up the 'go.mod' and 'go.sum' files."; false)
+fi