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.

vendor/github.com/sirupsen/logrus/go.mod

@@ -0,0 +1,10 @@
+module github.com/sirupsen/logrus
+
+require (
+	github.com/davecgh/go-spew v1.1.1 // indirect
+	github.com/konsorten/go-windows-terminal-sequences v1.0.1
+	github.com/pmezard/go-difflib v1.0.0 // indirect
+	github.com/stretchr/objx v0.1.1 // indirect
+	github.com/stretchr/testify v1.2.2
+	golang.org/x/sys v0.0.0-20180905080454-ebe1bf3edb33
+)