README: change semaphore CI status badge

We just reset Semaphore CI, and badge URL is changed.

README.md

@@ -2,7 +2,7 @@
 
 [![Go Report Card](https://goreportcard.com/badge/github.com/coreos/etcd)](https://goreportcard.com/report/github.com/coreos/etcd)
 [![Build Status](https://travis-ci.org/coreos/etcd.svg?branch=master)](https://travis-ci.org/coreos/etcd)
-[![Build Status](https://semaphoreci.com/api/v1/projects/406f9909-2f4f-4839-b59e-95082cb088f1/575109/badge.svg)](https://semaphoreci.com/coreos/etcd)
+[![Build Status](https://semaphoreci.com/api/v1/coreos/etcd/branches/master/shields_badge.svg)](https://semaphoreci.com/coreos/etcd)
 [![Docker Repository on Quay.io](https://quay.io/repository/coreos/etcd-git/status "Docker Repository on Quay.io")](https://quay.io/repository/coreos/etcd-git)
 
 **Note**: The `master` branch may be in an *unstable or even broken state* during development. Please use [releases][github-release] instead of the `master` branch in order to get stable binaries.