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/api-put-object-multipart.go

@@ -259,7 +259,7 @@ func (c Client) uploadPart(ctx context.Context, bucketName, objectName, uploadID
 
 	// Set encryption headers, if any.
 	customHeader := make(http.Header)
-	if sse != nil {
+	if sse != nil && sse.Type() != encrypt.S3 && sse.Type() != encrypt.KMS {
 		sse.Marshal(customHeader)
 	}