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/sys/unix/openbsd_unveil.go

@@ -0,0 +1,44 @@
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// +build openbsd
+
+package unix
+
+import (
+	"syscall"
+	"unsafe"
+)
+
+// Unveil implements the unveil syscall.
+// For more information see unveil(2).
+// Note that the special case of blocking further
+// unveil calls is handled by UnveilBlock.
+func Unveil(path string, flags string) error {
+	pathPtr, err := syscall.BytePtrFromString(path)
+	if err != nil {
+		return err
+	}
+	flagsPtr, err := syscall.BytePtrFromString(flags)
+	if err != nil {
+		return err
+	}
+	_, _, e := syscall.Syscall(SYS_UNVEIL, uintptr(unsafe.Pointer(pathPtr)), uintptr(unsafe.Pointer(flagsPtr)), 0)
+	if e != 0 {
+		return e
+	}
+	return nil
+}
+
+// UnveilBlock blocks future unveil calls.
+// For more information see unveil(2).
+func UnveilBlock() error {
+	// Both pointers must be nil.
+	var pathUnsafe, flagsUnsafe unsafe.Pointer
+	_, _, e := syscall.Syscall(SYS_UNVEIL, uintptr(pathUnsafe), uintptr(flagsUnsafe), 0)
+	if e != 0 {
+		return e
+	}
+	return nil
+}