Update go-checkpoint library and add link to README.

vendor/github.com/weaveworks/go-checkpoint/Makefile

@@ -0,0 +1,24 @@
+BUILD_IN_CONTAINER ?= true
+RM=--rm
+BUILD_IMAGE=golang:1.5.3
+
+ifeq ($(BUILD_IN_CONTAINER),true)
+
+all test:
+	$(SUDO) docker run $(RM) -ti \
+		-v $(shell pwd):/go/src/github.com/weaveworks/go-checkpoint \
+		-e GOARCH -e GOOS -e BUILD_IN_CONTAINER=false \
+		$(BUILD_IMAGE) make -C /go/src/github.com/weaveworks/go-checkpoint $@
+
+else
+
+all:
+	go get .
+	go build .
+
+test:
+	go get .
+	go test
+
+endif
+

vendor/github.com/weaveworks/go-checkpoint/checkpoint.go

@@ -4,9 +4,11 @@ package checkpoint
 
 import (
 	"crypto/rand"
+	"encoding/base64"
 	"encoding/binary"
 	"encoding/json"
 	"fmt"
+	"golang.org/x/crypto/scrypt"
 	"io"
 	"io/ioutil"
 	mrand "math/rand"
@@ -116,11 +118,15 @@ func Check(p *CheckParams) (*CheckResponse, error) {
 		p.OS = runtime.GOOS
 	}
 
-	// If we're given a SignatureFile, then attempt to read that.
+	// If we're not given a Signature, then attempt to read one.
 	signature := p.Signature
-	if p.Signature == "" && p.SignatureFile != "" {
+	if p.Signature == "" {
 		var err error
-		signature, err = checkSignature(p.SignatureFile)
+		if p.SignatureFile == "" {
+			signature, err = getSystemUUID()
+		} else {
+			signature, err = checkSignature(p.SignatureFile)
+		}
 		if err != nil {
 			return nil, err
 		}
@@ -145,6 +151,7 @@ func Check(p *CheckParams) (*CheckResponse, error) {
 	req.Header.Add("User-Agent", "HashiCorp/go-checkpoint")
 
 	client := cleanhttp.DefaultClient()
+	defer client.Transport.(*http.Transport).CloseIdleConnections()
 	resp, err := client.Do(req)
 	if err != nil {
 		return nil, err
@@ -154,6 +161,7 @@ func Check(p *CheckParams) (*CheckResponse, error) {
 	}
 
 	var r io.Reader = resp.Body
+	defer resp.Body.Close()
 	if p.CacheFile != "" {
 		// Make sure the directory holding our cache exists.
 		if err := os.MkdirAll(filepath.Dir(p.CacheFile), 0755); err != nil {
@@ -284,6 +292,23 @@ func checkResult(r io.Reader) (*CheckResponse, error) {
 	return &result, nil
 }
 
+// getSystemUUID returns the base64 encoded, scrypt hashed contents of
+// /sys/class/dmi/id/product_uuid.
+func getSystemUUID() (string, error) {
+	uuid, err := ioutil.ReadFile("/sys/class/dmi/id/product_uuid")
+	if err != nil {
+		return "", err
+	}
+	if len(uuid) <= 0 {
+		return "", fmt.Errorf("Empty system uuid")
+	}
+	hash, err := scrypt.Key(uuid, uuid, 16384, 8, 1, 32)
+	if err != nil {
+		return "", err
+	}
+	return base64.StdEncoding.EncodeToString(hash), nil
+}
+
 func checkSignature(path string) (string, error) {
 	_, err := os.Stat(path)
 	if err == nil {