Retina ready shields and icons

[diogomoretti]

Before:

After:

Note: SVGs from wikipedia, will be cached by github 👌

[insin]

I moved away from SVG badges because this is what they look like if you specify a reasonable minimum font size in your browser config:

I've opened https://github.com/badges/shields/issues/848 to see if there's something which can be done to fix this.