Readme: Use SVG badges.

Those badges look better on 2x (and more) displays. They're also a smaller download.

Note that the Travis CI badge was already sent as SVG.

I didn't put a bug number since all patches relating to this part of the code didn't (see eg. <https://github.com/mozilla/kuma/commit/a07203543f6506ed5ef06c74bd1248e08ad1ee8d>), and it seemed silly to file a bug form with questions such as "What would users do? What would happen as a result?" for this.

Full disclosure: I am <http://shields.io>'s maintainer. I talked to all parties involved in each of the badges shown.