Fix build status img in README to point only to master (#78)

Author: Alexander

README.md

@@ -1,4 +1,4 @@
-# go-git [![GoDoc](https://godoc.org/gopkg.in/src-d/go-git.v3?status.svg)](https://godoc.org/gopkg.in/src-d/go-git.v3) [![Build Status](https://travis-ci.org/src-d/go-git.svg)](https://travis-ci.org/src-d/go-git) [![codecov.io](https://codecov.io/github/src-d/go-git/coverage.svg)](https://codecov.io/github/src-d/go-git) [![codebeat badge](https://codebeat.co/badges/b6cb2f73-9e54-483d-89f9-4b95a911f40c)](https://codebeat.co/projects/github-com-src-d-go-git)
+# go-git [![GoDoc](https://godoc.org/gopkg.in/src-d/go-git.v3?status.svg)](https://godoc.org/gopkg.in/src-d/go-git.v3) [![Build Status](https://travis-ci.org/src-d/go-git.svg?branch=master)](https://travis-ci.org/src-d/go-git) [![codecov.io](https://codecov.io/github/src-d/go-git/coverage.svg)](https://codecov.io/github/src-d/go-git) [![codebeat badge](https://codebeat.co/badges/b6cb2f73-9e54-483d-89f9-4b95a911f40c)](https://codebeat.co/projects/github-com-src-d-go-git)
 
 A low level and highly extensible git client library for **reading** repositories from git servers.  It is written in Go from scratch, without any C dependencies.