Change Travis status to only show master branch status. (#865)

So that pull request build failure does not impact project status.
Signed-off-by: Yong Tang <yong.tang.github@outlook.com>

README.md

 # CoreDNS
 
 [![Documentation](https://img.shields.io/badge/godoc-reference-blue.svg?style=flat-square)](https://godoc.org/github.com/coredns/coredns)
-[![Build Status](https://img.shields.io/travis/coredns/coredns.svg?style=flat-square&label=build)](https://travis-ci.org/coredns/coredns)
+[![Build Status](https://img.shields.io/travis/coredns/coredns/master.svg?style=flat-square&label=build)](https://travis-ci.org/coredns/coredns)
 [![Code Coverage](https://img.shields.io/codecov/c/github/coredns/coredns/master.svg?style=flat-square)](https://codecov.io/github/coredns/coredns?branch=master)
 [![Go Report Card](https://goreportcard.com/badge/github.com/coredns/coredns?style=flat-square)](https://goreportcard.com/report/coredns/coredns)
 [![FOSSA Status](https://app.fossa.io/api/projects/git%2Bhttps%3A%2F%2Fgithub.com%2Fcoredns%2Fcoredns.svg?type=shield)](https://app.fossa.io/projects/git%2Bhttps%3A%2F%2Fgithub.com%2Fcoredns%2Fcoredns?ref=badge_shield)