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-stat.go

@@ -66,6 +66,9 @@ var defaultFilterKeys = []string{
 	"x-amz-bucket-region",
 	"x-amz-request-id",
 	"x-amz-id-2",
+	"Content-Security-Policy",
+	"X-Xss-Protection",
+
 	// Add new headers to be ignored.
 }