Switch to using SVG badges

This switches the DOI badges for GitHub from being low res PNGs to retina-friendly SVGs from [shields.io](http://shields.io).

For example, instead of looking like this:

![PNG](https://zenodo.org/badge/doi/10.5281/zenodo.12521.png)

They'll look like this:
![SVG](http://img.shields.io/badge/DOI-10.5281%2Fzenodo.12521-blue.svg)

This should help close badges/shields#176.