Update to use go modules (#671)

This includes updates to CI config, vendor files, etc.
I've hard-coded the k8s depedency at 1.13.4 to keep it inline with what
we currently have and to make sure a another run of `go mod tidy`
doesn't accidentally update it to an unexpected version.

Thanks to hectorj2f for carrying this along.

scripts/validate/dep.sh

@@ -1,17 +0,0 @@
-#!/usr/bin/env bash
-
-exit_code=0
-
-echo "==> Running dep check <=="
-
-dep check || exit_code=1
-
-if [ $exit_code -ne 0 ]; then
-  echo '`dep ensure` was not run. Make sure dependency changes are committed to the repository.'
-  echo 'You may also need to check that all deps are pinned to a commit or tag instead of a branch or HEAD.'
-  echo 'Check Gopkg.toml and Gopkg.lock'
-else
-  echo "dep check passed."
-fi
-
-exit $exit_code

scripts/validate/gomod.sh

@@ -0,0 +1,22 @@
+#!/usr/bin/env bash
+set -e
+
+exit_code=0
+
+echo "==> Running go mod tidy to add missing and remove unused modules <=="
+
+GO111MODULE=on go mod tidy || exit_code=1
+if [ $exit_code -ne 0 ]; then
+  echo 'Checking go.mod and go.sum files'
+else
+  echo "go mod tidy passed."
+fi
+
+git diff --exit-code go.mod go.sum vendor || exit_code=$?
+
+if [ ${exit_code} -eq 0 ]; then
+	exit 0
+fi
+
+echo "please run \`go mod tidy\` and check in the changes"
+exit ${exit_code}