Fix readme to specify git branch parameter of travis badge.

README.md

@@ -1,7 +1,7 @@
 go-libwebp
 ==========
 
-[![Build Status](https://travis-ci.org/harukasan/go-libwebp.svg)](https://travis-ci.org/harukasan/go-libwebp)
+[![Build Status](https://travis-ci.org/harukasan/go-libwebp.svg?branch=master)](https://travis-ci.org/harukasan/go-libwebp)
 [![GoDoc](https://godoc.org/github.com/harukasan/go-libwebp/webp?status.svg)](https://godoc.org/github.com/harukasan/go-libwebp/webp)
 
 A implementation of Go binding for [libwebp](https://developers.google.com/speed/webp/docs/api) written with cgo.