README: make badges render consistently (#1076)

It seems that the `<picture>` tags used previously hit a flaky bit of
GitHub rendering code and do not always display.
Making the badges be explicit hyperlinks to reasonable targets seems to
avoid this flaky path and display the badges consistently.
In future, we might like to upload small `.md` summaries to the Gist and
link those instead of the `.json` files.

Author: olupton