configure DNS for img.shields.io

[chadwhitacre]

@olivierlacan What do you want to block this on? At least until the URLs are revised (#19), eh? Though if we break backcompat there w/ /v1 then I suppose it doesn't hurt further to cut over sooner.

[chadwhitacre]

Eventually we'll want hosting to run through a Shields bank account (badges/shields#87).