use newer svg badge for coveralls

README.md

@@ -5,7 +5,7 @@ go-github is a Go client library for accessing the [GitHub API][].
 **Documentation:** [![GoDoc](https://godoc.org/github.com/google/go-github?status.svg)](https://godoc.org/github.com/google/go-github)  
 **Mailing List:** [go-github@googlegroups.com](https://groups.google.com/group/go-github)  
 **Build Status:** [![Build Status](https://travis-ci.org/google/go-github.png?branch=master)](https://travis-ci.org/google/go-github)  
-**Test Coverage:** [![Test Coverage](https://coveralls.io/repos/google/go-github/badge.png?branch=master)](https://coveralls.io/r/google/go-github?branch=master) ([gocov report](https://drone.io/github.com/google/go-github/files/coverage.html))
+**Test Coverage:** [![Test Coverage](https://coveralls.io/repos/google/go-github/badge.svg?branch=master)](https://coveralls.io/r/google/go-github?branch=master) ([gocov report](https://drone.io/github.com/google/go-github/files/coverage.html))
 
 go-github requires Go version 1.1 or greater.