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-get-options.go

@@ -44,7 +44,7 @@ func (o GetObjectOptions) Header() http.Header {
 	for k, v := range o.headers {
 		headers.Set(k, v)
 	}
-	if o.ServerSideEncryption != nil {
+	if o.ServerSideEncryption != nil && o.ServerSideEncryption.Type() != encrypt.S3 {
 		o.ServerSideEncryption.Marshal(headers)
 	}
 	return headers