Change BASE_URL to img.shields.io
Change BASE_URL to img.shields.io

It seems like a mistake from #870. Indeed, the code in that patch
defaults to shields.io for BASE_URL, but the author mentions they
think it defaults to img.shields.io: https://github.com/badges/shields/pull/870#discussion_r115143960
The correct value to maintain the behavior that was present prior
to the patch in question was indeed img.shields.io.

badges/shields,freezy-sk/shields,freezy-sk/shields,badges/shields,badges/shields