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/spf13/pflag/flag.go

@@ -925,13 +925,16 @@ func stripUnknownFlagValue(args []string) []string {
 	}
 
 	first := args[0]
-	if first[0] == '-' {
+	if len(first) > 0 && first[0] == '-' {
 		//--unknown --next-flag ...
 		return args
 	}
 
 	//--unknown arg ... (args will be arg ...)
-	return args[1:]
+	if len(args) > 1 {
+		return args[1:]
+	}
+	return nil
 }
 
 func (f *FlagSet) parseLongArg(s string, args []string, fn parseFunc) (a []string, err error) {
@@ -990,11 +993,12 @@ func (f *FlagSet) parseLongArg(s string, args []string, fn parseFunc) (a []strin
 }
 
 func (f *FlagSet) parseSingleShortArg(shorthands string, args []string, fn parseFunc) (outShorts string, outArgs []string, err error) {
+	outArgs = args
+
 	if strings.HasPrefix(shorthands, "test.") {
 		return
 	}
 
-	outArgs = args
 	outShorts = shorthands[1:]
 	c := shorthands[0]