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/golang.org/x/text/unicode/bidi/gen.go

@@ -26,7 +26,7 @@ func main() {
 }
 
 // bidiClass names and codes taken from class "bc" in
-// http://www.unicode.org/Public/8.0.0/ucd/PropertyValueAliases.txt
+// https://www.unicode.org/Public/8.0.0/ucd/PropertyValueAliases.txt
 var bidiClass = map[string]Class{
 	"AL":  AL,  // ArabicLetter
 	"AN":  AN,  // ArabicNumber