Update retryable dep

vendor/github.com/hashicorp/go-retryablehttp/client.go

@@ -192,19 +192,37 @@ func DefaultBackoff(min, max time.Duration, attemptNum int, resp *http.Response)
 
 // LinearJitterBackoff provides a callback for Client.Backoff which will
 // perform linear backoff based on the attempt number and with jitter to
-// prevent a thundering herd. The final backoff time is the attempt number
-// multiplied by the random chosen value between min/max.
+// prevent a thundering herd.
+//
+// min and max here are *not* absolute values. The number to be multipled by
+// the attempt number will be chosen at random from between them, thus they are
+// bounding the jitter.
+//
+// For instance:
+// * To get strictly linear backoff of one second increasing each retry, set
+// both to one second (1s, 2s, 3s, 4s, ...)
+// * To get a small amount of jitter centered around one second increasing each
+// retry, set to around one second, such as a min of 800ms and max of 1200ms
+// (892ms, 2102ms, 2945ms, 4312ms, ...)
+// * To get extreme jitter, set to a very wide spread, such as a min of 100ms
+// and a max of 20s (15382ms, 292ms, 51321ms, 35234ms, ...)
 func LinearJitterBackoff(min, max time.Duration, attemptNum int, resp *http.Response) time.Duration {
+	// attemptNum always starts at zero but we want to start at 1 for multiplication
+	attemptNum++
+
 	if max <= min {
-		// Unclear what to do here, so return min
-		return min
+		// Unclear what to do here, or they are the same, so return min *
+		// attemptNum
+		return min * attemptNum
 	}
+
 	// Seed rand; doing this every time is fine
 	rand := rand.New(rand.NewSource(int64(time.Now().Nanosecond())))
+
 	// Pick a random number that lies somewhere between the min and max and
 	// multiply by the attemptNum. attemptNum starts at zero so we always
 	// increment here.
-	return (min + time.Duration(int64(rand.Int63())%int64(max-min))) * time.Duration(attemptNum+1)
+	return time.Duration((rand.Int63() % int64(max-min)) * attemptNum)
 }
 
 // PassthroughErrorHandler is an ErrorHandler that directly passes through the

vendor/vendor.json

@@ -1141,10 +1141,10 @@
 			"revisionTime": "2018-03-31T00:25:53Z"
 		},
 		{
-			"checksumSHA1": "xCptnjI2GVTNvx+Td5enr9jvk4w=",
+			"checksumSHA1": "Zg/eHcBFPznCeqiRBQeV8WQw9G8=",
 			"path": "github.com/hashicorp/go-retryablehttp",
-			"revision": "0936e5acf1e32bbc3c210de331a1aefb3a2113bc",
-			"revisionTime": "2018-05-09T22:33:19Z"
+			"revision": "936ce55c28f313d8cf79061ffcb25a55bf631eb6",
+			"revisionTime": "2018-05-10T00:49:21Z"
 		},
 		{
 			"checksumSHA1": "A1PcINvF3UiwHRKn8UcgARgvGRs=",