Update build badge to report master branch only

The README build badge was reporting the last build overall, which showed the status from PRs on the main README even though they had nothing to do with the master branch.

Author: nelsam

README.md

@@ -1,6 +1,6 @@
 # Go Relational Persistence
 
-[![build status](https://img.shields.io/travis/go-gorp/gorp.svg)](http://travis-ci.org/go-gorp/gorp)
+[![build status](https://img.shields.io/travis/go-gorp/gorp.svg?branch=master)](http://travis-ci.org/go-gorp/gorp)
 [![code coverage](https://img.shields.io/coveralls/go-gorp/gorp.svg)](https://coveralls.io/r/go-gorp/gorp)
 [![issues](https://img.shields.io/github/issues/go-gorp/gorp.svg)](https://github.com/go-gorp/gorp/issues)
 [![godoc v1](https://img.shields.io/badge/godoc-v1-375EAB.svg)](https://godoc.org/gopkg.in/gorp.v1)