Merge pull request #2790 from nicolasnoble/using-jenkins-for-our-badge

Moving our build status badge from Travis to Jenkins.

README.md

-[![Build Status](https://travis-ci.org/grpc/grpc.svg?branch=master)](https://travis-ci.org/grpc/grpc)
+[![Build Status](https://grpc-testing.appspot.com/job/gRPC_master/badge/icon)](https://grpc-testing.appspot.com/job/gRPC_master)
 [![Coverage Status](https://img.shields.io/coveralls/grpc/grpc.svg)](https://coveralls.io/r/grpc/grpc?branch=master)
 
 [gRPC - An RPC library and framework](http://github.com/grpc/grpc)