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/fcntl.go

@@ -2,7 +2,7 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-// +build darwin dragonfly freebsd linux netbsd openbsd
+// +build dragonfly freebsd linux netbsd openbsd
 
 package unix
 
@@ -14,7 +14,11 @@ var fcntl64Syscall uintptr = SYS_FCNTL
 
 // FcntlInt performs a fcntl syscall on fd with the provided command and argument.
 func FcntlInt(fd uintptr, cmd, arg int) (int, error) {
-	valptr, _, err := Syscall(fcntl64Syscall, fd, uintptr(cmd), uintptr(arg))
+	valptr, _, errno := Syscall(fcntl64Syscall, fd, uintptr(cmd), uintptr(arg))
+	var err error
+	if errno != 0 {
+		err = errno
+	}
 	return int(valptr), err
 }