simplify

README.md

@@ -1,6 +1,9 @@
 # go-isatty
 
-[![Build Status](https://travis-ci.org/mattn/go-isatty.svg?branch=master)](https://travis-ci.org/mattn/go-isatty) [![Coverage Status](https://coveralls.io/repos/github/mattn/go-isatty/badge.svg?branch=master)](https://coveralls.io/github/mattn/go-isatty?branch=master) [![Go Report Card](https://goreportcard.com/badge/mattn/go-isatty)](https://goreportcard.com/report/mattn/go-isatty)
+[![Godoc Reference][godoc-img]][godoc]
+[![Build Status][ci-img]][ci]
+[![Coverage][coveralls-img]][coveralls]
+[![Go Report Card][goreportcard-img]][goreportcard]
 
 isatty for golang