Update circleCI badge to more consistent shield.

The shield badge matches the go report and godoc badges.

Signed-off-by: RPSeq <ryan.smith.p@gmail.com>

README.md

@@ -1,6 +1,6 @@
 # Helm
 
-[![CircleCI](https://circleci.com/gh/helm/helm.svg?style=svg)](https://circleci.com/gh/helm/helm)
+[![CircleCI](https://circleci.com/gh/helm/helm.svg?style=shield)](https://circleci.com/gh/helm/helm)
 [![Go Report Card](https://goreportcard.com/badge/github.com/helm/helm)](https://goreportcard.com/report/github.com/helm/helm)
 [![GoDoc](https://godoc.org/k8s.io/helm?status.svg)](https://godoc.org/k8s.io/helm)