Use svg instead of png to get better image quality

README.md

-[Grafana](http://grafana.org) [![Build Status](https://api.travis-ci.org/grafana/grafana.png)](https://travis-ci.org/grafana/grafana) [![Coverage Status](https://coveralls.io/repos/grafana/grafana/badge.png?branch=develop)](https://coveralls.io/r/grafana/grafana?branch=develop)
+[Grafana](http://grafana.org) [![Build Status](https://api.travis-ci.org/grafana/grafana.svg)](https://travis-ci.org/grafana/grafana) [![Coverage Status](https://coveralls.io/repos/grafana/grafana/badge.png?branch=develop)](https://coveralls.io/r/grafana/grafana?branch=develop)
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