Add michagogo to gitian download scripts

I've been gitian-building lately, and from conversation in #bitcoin-dev on freenode it sounded like people doing that should be listed here. I understand if I'm not well-known enough to be included. If you don't want to include this, that's fine with me. In addition, if you'd like more verification or confirmation of who I am, I'm willing to provide information about my identity to the core dev team upon request.

As I realize I'm not as well-known as the other users on the list, I've weighted myself lower as well.