Cannot composer dump-autoload - file put contents error?

I'm trying to:

composer dump-autoload

On a Ubuntu 16.04 server.

I get the error:

 file_put_contents(/var/www/http://ift.tt/2dGxRfs): failed to open stream: Permission denied

I have:

1. Change permission of storage dir to 755
2. Made sure the www-data user has correct usage rights

I still get the error. Any ideas on a fix?

via Chebli Mohamed