Update dependencies, enable pruning for vendor/

So, `dep` got an nice new feature to remove tests and non-go files from
`vendor/`, and this brings the size of the vendor directory from ~300MiB
down to ~20MiB. We don that now.

vendor/github.com/minio/minio-go/retry.go

@@ -111,6 +111,9 @@ func isNetErrorRetryable(err error) bool {
 			} else if strings.Contains(err.Error(), "connection timed out") {
 				// If err is a net.Dial timeout, retry.
 				return true
+			} else if strings.Contains(err.Error(), "net/http: HTTP/1.x transport connection broken") {
+				// If error is transport connection broken, retry.
+				return true
 			}
 		}
 	}
@@ -128,6 +131,7 @@ var retryableS3Codes = map[string]struct{}{
 	"InternalError":         {},
 	"ExpiredToken":          {},
 	"ExpiredTokenException": {},
+	"SlowDown":              {},
 	// Add more AWS S3 codes here.
 }