Use Shields.io style badge for Circle CI

Bit less visually noisy, and clearer.

Signed-off-by: Olivier Lacan <[email protected]>

README.md

@@ -1,7 +1,8 @@
 <p align="center"><img src="docs/logo_and_name.png" width="600" alt="Loki Logo"></p>
 
 <p align="center">
-<a href="https://circleci.com/gh/grafana/loki/tree/master"><img src="https://circleci.com/gh/grafana/loki.svg?style=svg&circle-token=618193e5787b2951c1ea3352ad5f254f4f52313d" alt="CircleCI" /></a>
+
+<a href="https://circleci.com/gh/grafana/loki/tree/master"><img src="https://circleci.com/gh/grafana/loki.svg?style=shield&circle-token=618193e5787b2951c1ea3352ad5f254f4f52313d" alt="CircleCI" /></a>
 <a href="https://goreportcard.com/report/github.com/grafana/loki"><img src="https://goreportcard.com/badge/github.com/grafana/loki" alt="Go Report Card" /></a>
 <a href="http://slack.raintank.io/"><img src="https://img.shields.io/badge/join%20slack-%23loki-brightgreen.svg" alt="Slack" /></a>
 </p>