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.go

@@ -99,7 +99,7 @@ type Options struct {
 // Global constants.
 const (
 	libraryName    = "minio-go"
-	libraryVersion = "5.0.0"
+	libraryVersion = "v6.0.5"
 )
 
 // User Agent should always following the below style.