Attempt to prevent security warnings when using HTTPS

[miketheman]

When visiting a page that displays external images, such as badges, a security warning is displayed to the user via a yellow triangle on the SSL badge. See https://supermarket.getchef.com/cookbooks/apt for an example.
The badges for this page are at http://img.shields.io/* and can definitely be rewritten to https on the backend.
This will not cover/handle all cases, so possibly one solution is to attempt to rewrite the header to https, and display a "insecure image" instead, or some other solution.

[brettchalupa]

@miketheman good catch & suggestions, I have added them to the Supermarket Trello board: https://trello.com/c/MOZhgNZ7

[sethvargo]

https://github.com/atmos/camo

[raskchanky]

The fix for this was merged and will go to production soon.