Switch readme badges to svg

SVG readme badges are crisper :)

Before:
![screen shot 2014-11-25 at 14 33 43](https://cloud.githubusercontent.com/assets/162976/5177616/54833ca4-74b0-11e4-9ea0-4fdcff724695.png)

After:
![screen shot 2014-11-25 at 14 35 05](https://cloud.githubusercontent.com/assets/162976/5177617/5a8c138c-74b0-11e4-989d-59cbe03aa9ca.png)

This PR inspired by postmodern/ruby-install#191 and @parndt