Replace .png badges with .svg

Makes the badges much nicer on hi-dpi devices (phones, retina MBP).

BTW [CodeClimate measures test coverage too](http://blog.codeclimate.com/blog/2013/09/11/test-coverage-and-code-quality-better-together/), so you could move there from coveralls.
Merge pull request #197 from mrfoto/patch-1

Replace .png badges with .svg